Pressure sensor evaluation for respiratory apparatus

ABSTRACT

A respiratory apparatus evaluates accuracy of a pressure sensor, such as when only a single pressure sensor is provided. The accuracy of the pressure sensor may be assessed based on pressure measurement obtained from the pressure sensor and a subordinate or secondary characteristic of the respiratory device such as altitude or atmospheric pressure. A controller or processor may calculate the altitude of the respiratory device based in part on the pressure measurement. In some embodiments, the assessment of the pressure sensor may involve an evaluation of the calculated altitude. In some cases, the assessment of the pressure sensor may involve determining an estimated pressure based on a calculated altitude, and comparing the pressure measurement obtained from the pressure sensor with the estimated pressure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/735,860 filed on Jan. 7, 2020, which is a continuation of U.S. patentapplication Ser. No. 14/411,769 filed on Dec. 29, 2014, which is anational phase entry under 35 U.S.C. § 371 of International ApplicationNo. PCT/AU2013/000695 filed Jun. 27, 2013, published in English, whichclaims priority from U.S. Provisional Patent Application No. 61/666,367filed Jun. 29, 2012, all of which are incorporated herein by reference.

FIELD OF TECHNOLOGY

The present technology relates to methods for monitoring accuracy ofsensors used for respiratory apparatus, such as methods and apparatusfor determining accuracy of a pressure sensor.

BACKGROUND OF THE TECHNOLOGY

Patients diagnosed with disordered breathing problems, such as sleepdisordered breathing, snoring, upper airway flow limitation, hypopnea,apnea, or similar, may rely on a respiratory apparatus, such as acontinuous positive airway pressure (CPAP) or a BiLevel positivepressure device, to assist with maintaining an open airway. Other formsof a respiratory apparatus include a ventilator. A ventilator assists apatient to breathe by providing oxygen to the lungs and removing carbondioxide from the body. A ventilator can be used for a patient that isunable to or has a reduced capacity to, for example due to lung disease,breathe on their own.

A respiratory apparatus may include a flow generator coupled to apatient interface (e.g., mask or cannula, etc.) to generate a supply ofpositively pressurized gas for supply to the patient's airways at one ormore pressures.

A respiratory apparatus may also include one or more sensors, such as apressure sensor, to monitor characteristics of the breathable gas, suchas pressure, to be provided to the patient. Based on the measuredcharacteristics, the apparatus may control adjustments to respiratoryparameters such as by changing a treatment pressure above atmosphericpressure to splint open an obstructed or partially obstructed patientairway or a change in a pressure support (PS) to provide ventilationthat meets a target volume where pressure support is a differencebetween an inspiratory pressure and an expiratory pressure. Suchadjustments may be made by a controller setting or adjusting the motorspeed of the flow generator or the controller setting of an aperture ofa relief valve of the system. A false reading by any sensor involved inthe control of the breathable gas may yield inaccurate control thereof,which, in turn, may adversely affect patient breathing, comfort orsafety.

To ensure accurate performance by a respiratory apparatus, it may bedesirable to develop methods for monitoring or detecting the accuracy ofsensors, before and/or during use, such as a pressure sensor, or fordetecting a fault with such sensors.

BRIEF SUMMARY OF THE TECHNOLOGY

Some embodiments of the present technology include a methodology tomonitor accuracy of a sensor involved in control of a respiratorytreatment.

Some embodiments of the present technology include a respiratoryapparatus that monitors accuracy of a sensor involved in its control ofthe apparatus.

Some embodiments may evaluate a pressure sensor based on a subordinateor secondary characteristic of the apparatus.

Some embodiments may evaluate a pressure sensor based on an altitude ofthe apparatus.

Some embodiments may evaluate a pressure sensor based on an atmosphericpressure, such as an atmospheric pressure, of the apparatus or in whichthe apparatus is operated.

Some such embodiments may achieve such monitoring without redundantsensors. For example, some embodiments may evaluate a pressure sensorwithout an additional pressure sensor. Some embodiments may evaluate apressure sensor without an altimeter.

Some embodiments of the present technology include a method fordetermining accuracy of a pressure sensor in a respiratory device. Themethod may include measuring a pressure of a flow of breathable gasgenerated by the respiratory device using the pressure sensor. Themethod may also include determining, with a processor, accuracy of thepressure sensor based on the measured pressure and an altitude of therespiratory device.

In some cases, the respiratory device may include a flow generator witha motor included therein to generate a pressurised flow of breathablegas. The altitude of the respiratory device may be input by a user. Thealtitude of the respiratory device may be measured by an altimeter ofthe respiratory device. In some cases, the processor may calculate anestimate of the altitude of the respiratory device. Moreover, theprocessor may determine the accuracy of the pressure sensor based on anevaluation of the calculated altitude.

In some cases, the processor may calculate the altitude of therespiratory device as a function of (a) the pressure measured by thepressure sensor and one or both of (b)(1) a measured flow rate of theflow of breathable gas and (b)(2) a measured motor speed of the flowgenerator. Optionally, the processor may calculate the altitude of therespiratory device as a function of a measured temperature of the flowof breathable gas.

The processor may calculate the altitude of the respiratory device whenthe flow generator controls the gas at a constant predetermined flowrate, such as about 20 liters/minute, or a constant predetermined flowrate that is less than 50 liters/minute, or a constant predeterminedflow rate that is in the range between about 10 liters/minute and about60 liters/minute. Still further, the processor may calculate thealtitude of the respiratory device when the flow generator controls aconstant predetermined motor speed.

In some cases, the processor may evaluate the calculated altitude bycomparing the calculated altitude with a predetermined range ofaltitudes. In some cases, the predetermined range of altitudes may bebetween 0 and 9000 feet above sea level. The predetermined range ofaltitudes may be between 500 feet below sea level and 10,000 feet abovesea level. The processor may deem the calculated altitude acceptablewhen the calculated altitude is within the predetermined range ofaltitudes. The processor may deem the calculated altitude unacceptablewhen the calculated altitude is outside the predetermined range ofaltitudes.

In some cases, the processor may calculate the altitude of therespiratory device at a predetermined frequency over a predeterminedperiod of time. The predetermined frequency may be between about 1 and 2hertz. The predetermined period of time may be about 5 seconds.Optionally, the processor may evaluate the pressure sensor based on anaverage of the calculated altitudes. The processor may even determinethe pressure sensor to be accurate when an average of the calculatedaltitudes satisfies a threshold comparison. The processor may evaluatethe pressure sensor in an initialization process before the respiratorydevice provides treatment to a patient. In some cases, the processor maystore the altitude of the respiratory device in a memory.

In some versions, the processor may evaluate accuracy of the pressuresensor by calculating expected pressure of the gas generated by therespiratory device and by comparing the measured pressure with theexpected pressure. The processor may calculate the expected pressurefrom the altitude of the respiratory device, a measured flow rate of theflow of breathable gas, and a measured motor speed of the flowgenerator. The processor may calculate the expected pressure from ameasured temperature of the flow of breathable gas. The processor maydetermine the accuracy of the pressure sensor by comparing a differencebetween the measured pressure and the expected pressure with apredetermined threshold. The predetermined threshold may be about 5cmH₂O. The processor may determine the pressure sensor inaccurate whenthe difference exceeds the predetermined threshold. The processor maydetermine the pressure sensor to be accurate when the difference iswithin the predetermined threshold. In some cases, the measurement ofthe pressure, the calculation of the expected pressure, and thecomparison of the measured pressure with the expected pressure may beperformed at a predetermined frequency over a predetermined period oftime. The predetermined frequency may be between about 1 and 2 hertz.The predetermined period of time may be about 5 seconds. In some cases,the processor may determine the pressure sensor to be inaccurate basedon a plurality of comparisons between measured pressures and expectedpressures.

In some cases, the altitude of the respiratory device may be a firstaltitude, and the processor may determine the accuracy of the pressuresensor by calculating a second altitude of the respiratory device and bycomparing the second altitude of the respiratory device with the firstaltitude of the respiratory device. The processor may calculate thesecond altitude of the respiratory device from a measured flow rate ofthe flow of breathable gas, a measured motor speed of the flowgenerator, and the pressure measured by the pressure sensor. Theprocessor may calculate the second altitude of the respiratory devicefrom a measured temperature of the flow of breathable gas. The processormay determine the accuracy of the pressure sensor by comparing adifference between the first altitude and the second altitude with apredetermined threshold. The predetermined threshold may be about 600feet, for example. The processor may determine the pressure sensor to beinaccurate when the difference exceeds the predetermined threshold. Theprocessor may determine the pressure sensor to be accurate when thedifference is within the predetermined threshold. The processor maycalculate the second altitude at a predetermined frequency for apredetermined period of time. The predetermined frequency may be betweenabout 1 and 2 hertz. The predetermined period of time may be about 5seconds.

In some such cases, the processor may determine the pressure sensorinaccurate when an average of the second altitude calculated during thepredetermined period of time differs from the first altitude by anoffset greater than a predetermined threshold. The processor maydetermine the pressure sensor accurate when an average of the secondaltitude calculated during the predetermined period of time differs fromthe first altitude by an offset not greater than a predeterminedthreshold.

In some versions, the method may include setting a motor speed for theflow generator based on the measured pressure, when the processordetermines the pressure sensor inaccurate. The method may also includemaintaining the motor speed under a speed limit threshold. The methodmay also include determining a desired gas pressure to be generated bythe flow generator, and determining a desired motor speed for the flowgenerator based on a predetermined association between motor speedvalues and pressure values.

Some embodiments of the present technology include a respiratoryapparatus. The apparatus may include a flow generator with a blowerincluded therein to generate a breathable gas for a patient interface ata pressure above atmospheric pressure. A pressure sensor may be coupledwith the flow generator and may be configured to measure pressure of theflow of breathable gas. The apparatus may also include a processorcoupled with the pressure sensor and configured to determine accuracy ofthe pressure sensor based on the measured pressure and an altitude ofthe respiratory apparatus.

The respiratory apparatus may also include a flow sensor configured tomeasure a flow rate of the flow of breathable gas. The respiratoryapparatus may also include a motor speed sensor configured to measure amotor speed of the flow generator. The respiratory apparatus may alsoinclude a user input/output (I/O) device configured to receive thealtitude of the respiratory apparatus input by a user. The respiratoryapparatus may also include an altimeter to determine the altitude of therespiratory apparatus.

The processor may be configured to set the flow generator to a motorspeed based on the pressure measured by the pressure sensor, when theprocessor determines the pressure sensor inaccurate. The processor maybe configured to maintain the motor speed under a speed limit threshold.The processor may be configured to determine a desired gas pressure tobe generated by the flow generator, and to determine a desired motorspeed for the flow generator based on a predetermined associationbetween motor speed values and pressure values.

The subject headings used in the detailed description are included onlyfor the ease of reference of the reader and should not be used to limitthe subject matter found throughout the disclosure or the claims. Thesubject headings should not be used in construing the scope of theclaims or the claim limitations.

Various aspects of the described example embodiments may be combinedwith aspects of certain other example embodiments to realize yet furtherembodiments. It is to be understood that one or more features of any oneexample may be combinable with one or more features of the otherexamples. In addition, any single feature or combination of features inany example or examples may constitute patentable subject matter.

Other features of the technology will be apparent from consideration ofthe information contained in the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the presentdisclosure and, together with the detailed description of theembodiments given below, serve to explain the principles of thedisclosure.

FIG. 1 shows an example respiratory apparatus of the present technology;

FIG. 2 is a block diagram of an example configuration of the respiratoryapparatus of FIG. 1 with optional components;

FIG. 3 is a flow diagram illustrating an example methodology for acontroller to determine accuracy of operation of a pressure sensor;

FIG. 4 is a block diagram of an example configuration of a pressuresensor fault detector;

FIG. 5 is a flow diagram illustrating another methodology for acontroller to determine accuracy of a pressure sensor;

FIG. 6 is a flow diagram illustrating a method for determining accuracyof the pressure sensor according to another embodiment;

FIG. 7 is a flow diagram illustrating a method for determining accuracyof the pressure sensor according to yet another embodiment;

FIG. 8 is a flow diagram illustrating a method for determining accuracyof the pressure sensor according to still yet another embodiment;

FIG. 9 is a flow diagram illustrating a method for determining accuracyof the pressure sensor according to a further embodiment;

FIG. 10 is a flow diagram illustrating a method for determining accuracyof the pressure sensor according to a yet further embodiment;

FIG. 11 is a flow diagram illustrating a method for determining accuracyof the pressure sensor according to a still yet further embodiment;

FIG. 12 is a flow diagram of a method by a pressure sensor fault handleraccording to one embodiment; and

FIG. 13 is a flow diagram of a method by a pressure sensor fault handleraccording to another embodiment.

DETAILED DESCRIPTION

For simplicity and clarity of illustration, like reference numerals maybe used in the drawings to identify identical or analogous structuralelements.

Flow diagrams may be used in the drawings to illustrate processes,operations or methods performed by components, devices, parts, systems,or apparatuses disclosed herein. The flow diagrams are mere exemplaryillustrations of steps or algorithms performed in individual processes,operations or methods, such as by a suitably configured controller orprocessor. The processes may be performed in the precise order asillustrated in the flow diagrams. Alternatively, various steps may behandled simultaneously or performed in sequences different from thatillustrated. Steps may also be omitted from or added to the flowdiagrams unless otherwise stated.

Example System Components

As illustrated in FIG. 1 , example embodiments of the present technologymay include a respiratory apparatus 100. The respiratory apparatus 100may be any type of a respiratory apparatus including, but not limitedto: a continuous positive airway pressure (CPAP) device, an automaticpositive airway pressure (APAP) device, a bi-level positive airwaypressure (BPAP) device, a variable positive airway pressure (VPAP)device, and a ventilator.

FIG. 2 is a block diagram illustrating an example bus configuration ofthe respiratory apparatus 100 shown in FIG. 1 . The apparatus 100 mayinclude one or more of the following components: blower 130, a pressuresensor 104 (which may be located proximate to the mask or proximate to ablower or in both locations), a flow sensor 106, a motor speed sensor108, a temperature sensor 110, an altimeter 112, user I/O devices 114, aprocessor 116, a memory 118, a pressure sensor fault detector 120, and apressure sensor fault handler 122 and optionally a second pressuresensor 124. One or more of these components may be operatively connectedwith each other via wireless communication, physical coupling and/orelectrical coupling, such as with a bus 126. One of more of thesecomponents may transmit or receive executable instructions in analog ordigital signals to or from one or more of other components. Details withrespect to each component are given below.

The flow generator 102 may be configured to generate breathable gas froma blower 130 (e.g., a motor and impeller) to a patient via a patientinterface 132. The blower 130 may be a servo-controller blower such as ablower in a volute. An output of the flow generator 102 may be coupledto a patient interface 132, and a gas delivery conduit 134 that directsgenerated breathable gas between the blower 130 and the patientinterface 132.

The flow generator 102 may be configured to generate breathable gas atdifferent therapeutic pressure levels or flows. The flow generator 102,with a suitable controller, may adjust the blower 130 output by varyingthe current or voltage supplied to the motor. As discussed in moredetail herein, the control of the operation of the blower 130 may be viaa pressure control loop (i.e., control of measured pressure to satisfy atarget/desired pressure set point), a flow control loop (i.e., controlof measured flow to satisfy a target/desired flow set point) and/or aspeed control loop (i.e., control of measured speed of the blower'smotor to meet a target/desired speed set point).

Thus, the apparatus 100 may include a variety of sensors to observe ordetect characteristics of the gas within the flow generator 102. Theapparatus 100 may rely on readings provided by one or more of somesensors to determine accuracy of one or more of other sensors. Detailsof such monitoring with each sensor are given below.

For example, the apparatus 100 may include a pressure sensor 104 todetect pressure of the gas delivered by the flow generator 102. Thepressure sensor 104 may be a pressure transducer that may produce apressure signal proportional to a gas pressure associated with operationor use of the apparatus 100. The pressure sensor 104 may be positionedto measure the gas pressure at various positions with respect to theflow generator 102, such as, at the patient interface 132, at an outletof the flow generator 102, or at the gas delivery conduit 134 orcombinations thereof. For instance, as illustrated in FIG. 1 , thepressure sensor 104 may be positioned at the patient interface 132, andin turn, may measure the gas pressure at the patient interface 132.Alternatively, the pressure sensor 104 may be positioned in the conduit134, and in turn, may detect the gas pressure within the conduit 134.The pressure sensor 104 may also be positioned at the outlet of the flowgenerator 102, such that the pressure sensor 104 may measure thepressure of the gas immediately generated by the flow generator 102.Optionally, the pressure measurements may be adjusted to account forpressure drop within the system such as for estimating mask pressurewhen a pressure sensor measures pressure proximate to the outlet of theblower 130 of the flow generator 102. In such an arrangement a pressuredrop along the air delivery tube may be predefined, estimated orcalculated.

Optionally, the apparatus 100 may include a flow sensor 106 to detectflow, such as the flow attributable to the blower 130, any system ormask leak and/or patient respiratory flow. The flow sensor 106 mayinclude a pneumotachograph, a differential pressure transducer, or othersimilar devices to produce a flow signal or a flow reading representingthe flow rate.

With continued reference to FIG. 1 , the apparatus 100 may include amotor speed sensor 108 to detect the motor speed, e.g., the rotationalspeed, of the motor in the flow generator 102. The motor speed sensor108 may include a Hall effect sensor or similar. A further example of amotor speed sensor 108 suitable for the present technology may be foundin PCT/AU2005/001688 filed on Nov. 2, 2005, the entire disclosure ofwhich is incorporated herein by reference. In some embodiments, bymonitoring the motor speed and, optionally, the current supplied to themotor, a controller of the apparatus 100 may estimate the flow rate orthe gas pressure.

In one embodiment, the apparatus 100 may optionally include atemperature sensor 110 to measure temperature of the gas delivered bythe flow generator 102. The temperature sensor 110 may include atemperature transducer, such as a thermocouple or a resistancetemperature detector (RTD). Depending on the placement of thetemperature sensor 110, the temperature sensor 110 may measure the gastemperature at alternative positions within the flow generator 102. Inone embodiment, as illustrated in FIG. 1 , the temperature sensor 110may be positioned at the blower 130 outlet, such as near the flow sensor106, of the flow generator 102, and in turn, may measure the temperatureof the gas immediately exiting the blower 130 of the flow generator 102.Alternatively the temperature sensor 110 may be positioned upstream ofthe blower, such as at the blower inlet or external to apparatus 100 tomeasure the temperature of the gas entering the blower or the ambienttemperature. The temperature of the gas leaving the blower may then beestimated or calculated based on this measured inlet or ambienttemperature.

With continued reference to FIG. 1 , in some embodiments, the apparatus100 may include an altimeter 112. The altimeter 112 may detect altitudeof the apparatus 100, such as the altitude relative to a fixed level,e.g., sea level. The altimeter 12 may be positioned at any suitablelocation on or within the apparatus 100.

The apparatus 100 may optionally include user I/O devices 114 tofacilitate user operation of the apparatus 100. A user of the apparatus100 may be a physician, nurse, clinician, caretaker or patient. The userI/O devices 114 may include one or more of user input devices including,but not limited to, a keyboard, touch panel, control buttons, mouse, andswitch. For example, such an I/O device may be implemented to acceptinput entered by a user such as an altitude of the apparatus 100.

The user I/O devices 114 may include a user output device, such as adisplay or alarms (not shown). The display may be a monitor or LCDpanel. The display may display information regarding the status of theapparatus 100, e.g., the pressure of the gas delivered to the patient asobtained from the pressure sensor 104. Alternatively, the display mayalso display a status or warning message concerning accuracy or faultfollowing testing of the operation of a sensor. The alarms may provide awarning alert via a sound and/or light e.g. LED light, to identify afault condition.

In addition to the various sensors described above, the apparatus 100may include a controller, such as a processor 116, to control operationof the processes related to the apparatus 100. The processor 116 mayrefer to a single processor or a collection of processors including oneor more of the following: central processing unit (CPU), microprocessor,digital signal processor, front end processor, coprocessor, dataprocessor, and/or analog signal processor. The processor 116 may beimplemented with one or more application specific integrated circuits(ASICs). In one aspect, the apparatus 100 as a whole may have aprocessor 116 to perform acts of each component described herein inaccordance with programmable instructions executed by the processor 116.Alternatively, one or more individual components of the apparatus 100,e.g., the pressure sensor fault detector 120 and the pressure sensorfault handler 122, may each have its own processor configured to executecomponent-specific instructions.

In some aspects, the processor 116 may be physically mounted within theapparatus 100. Alternatively, the processor 116 may be located remotelyfrom the apparatus 100, and may communicate with the apparatus 100 via anetwork (not shown). When there are a collection of processors, one ormore processors 116 may be physically mounted within the apparatus 100,while the remaining processors may communicate remotely with theapparatus 100 via a network.

In one embodiment, the apparatus 100 may include a memory 118 to storeprogrammable instructions executable by the processor 116. The memory118 may include a volatile memory, a non-volatile memory, or acombination thereof. The volatile memory may include a RAM, such as adynamic random access memory (DRAM) or static random access memory(SRAM), or any other forms of alterable memory that may be electricallyerased and reprogrammed. The non-volatile memory may include a ROM, aprogrammable logical array, or other forms of non-alterable memory whichcannot be modified, or can be modified only slowly or with difficulty.The non-volatile memory may include firmware.

Further, embodiments of the present technology may include one or moredetectors to assess accuracy of one or more sensors described above, ordetermine if a fault has occurred in one or more sensors. For instance,the apparatus 100 may include a pressure sensor fault detector 120 thatimplements algorithms to assess accuracy of the pressure sensor 104. Thealgorithms may be implemented in programmable instructions executable bythe processor 116. The programmable instructions may be stored in thememory 118, such as firmware, or a data storage (not shown) of theapparatus 100 or may otherwise be implemented as one or more ASICs.

Sensor Evaluation

To evaluate accuracy of the pressure sensor 104, the detector 120 mayreceive measurements from one or more of the sensors 104-112 describedabove. One or more of the sensors 104-112 may provide measurements tothe detector 120 in real time or quasi-real time, or on demand.

FIG. 3 is a flowchart 300 illustrating a suitable process performed bythe detector 120 to evaluate accuracy of the pressure sensor 104.Although steps discussed herein are made in reference to the detector120, one or more of these steps may be performed by the processor 116.

At 302, the detector 120 may obtain a pressure measure from the pressuresensor 104. At 304, the detector 120 may evaluate accuracy of thepressure sensor 104 based on the pressure measure and an altitude of theapparatus 100. The altitude of the apparatus 100 may include one or moreof (1) a previously calculated altitude estimate from a prior pressuremeasure of the apparatus 100, (2) an expected altitude or altitude rangefor the apparatus 100, (3) an actual or expected altitude input into theapparatus 100 for operation; and/or (4) an altitude measured by analtimeter 112 of the apparatus 100. The detector 120 may calculate analtitude estimate based on the pressure measure, and compare thealtitude estimate to any one or more of the altitudes (1)-(4) describedabove.

Optionally, the detector 120 may evaluate the accuracy of the pressuresensor 104 by comparing the pressure measure and a calculated estimateof pressure that is based on a previously calculated altitude estimate.

Based on the comparison, the detector 120 may determine whether theoperation of the sensor 104 is accurate or faulty. For example, if thecompared values are identical or substantially similar to each other,the sensor 104 may be deemed accurate. By contrast, if the comparedvalues are different or substantially different, the sensor may bedeemed faulty.

Various example embodiments of the detector 120 and its processes arediscussed in more detail herein with reference to FIGS. 4-12 .

FIG. 4 is a block diagram 400 illustrating an example configuration ofthe detector 120. In some embodiments, the detector 120 may include oneor more of the following components: an altitude estimation unit 402, analtitude validation unit 404, a pressure estimation unit 406 and apressure validation unit 408. One or more of these units 402-408 mayobtain measurements from one or more of the sensors 104-112: namelypressure sensor 104, temperature sensor 110; motor speed sensor 108;flow sensor 106; and altimeter 112. One or more of these units 402-408may evaluate an accuracy of the pressure sensor 104 based on themeasurements obtained from one or more of the sensors 104-112. Exampleoperations with respect to each unit are as follows.

The altitude estimation unit 402 may estimate altitude of the apparatus100 based on measurements obtained from one or more of the sensors104-110: pressure sensor 104, temperature sensor 110; motor speed sensor108; and flow sensor 106.

In one embodiment, the altitude estimation unit 402 may calculate anestimate of the altitude during operation of the apparatus 100 as afunction of some or all of the following: the measured pressure of thegas obtained from the pressure sensor 104, the measured flow rate of thegas obtained the flow sensor 106, the measured temperature obtained fromthe temperature sensor 110, and the measured motor speed obtained fromthe motor speed sensor 108. For example, the altitude estimation unit402 may compute the altitude of the apparatus 100 according to thefollowing function:

h=ƒ _(x)(P,T and ω)

-   -   where:    -   h is the estimated altitude;    -   P is the measure of pressure;    -   T is an optional measure of temperature that in some embodiments        may be omitted; and    -   ω is a measure of motor speed.

ƒ_(x) is a polynomial function (e.g., y=Ax²+Bx+c of a suitable degreedepending on the constants and parameters of the function) applied tothe measured values that may be derived empirically, such as by a GrayBox Model or best fit model using suitable test data. In a case where ameasure of flow is not implemented in the function as listed in theexample above, the measurements applied to the function may be obtainedduring operation of the apparatus 100 while a controller, e.g., theprocessor 116, of the apparatus 100 controls the motor of the blower 130to deliver a constant flow rate such as by controlling the blower 130 bya flow control loop and setting a fixed flow set point. An example flowset point may be in a range from 10 to 60 liters/minute, such as a flowrate set point of 20 liters/minute. Such a fixed operation mayoptionally be performed by the apparatus 100 as a start-up orpre-treatment procedure and may take place while the apparatus 100 isnot being used by a patient/user (i.e., without the patient interface132 on the patient). Such a fixed operation may serve to simplify theparameters of the function for calculating the altitude. Alternatively,in some embodiments, the measurement operation may be conducted whilethe detector 120 controls a constant motor speed rather than controllinga constant flow rate. In such a case, the measured flow may be utilizedin the above function rather than the measured motor speed, though bothmay be utilized.

From Fan Law for a centrifugal blower, pressure P at the outlet at zeroflow is given by:

$P = {\frac{\rho\omega^{2}}{2}\left( {r_{bladeend}^{2} - r_{bladestart}^{2}} \right)}$

-   -   ω is motor speed (e.g., rotational speed);    -   ρ is air density;    -   r_(bladeend) is the radius of the circle traced by the end of        the impeller blade; and    -   r_(bladestart) is the radius of the circle traced by the start        of the impeller blade.

Assume an empirical form for pressure derived from the Fan Law above atconstant flow as:

P≅φρω ²+(δ+θh)ω

Where φ, δ and θ are constants.

Typically centrifugal blowers have dropping fan curves (as seen inliterature) hence pressure at any flow (Q) can be approximated as:

P≅α·(φ₂ ·Q ²+φ₁ ·Q+φ ₀)ρ·ω²+[(δ₂ ·Q ²+δ₁ ·Q+δ ₀)+(θ₂ ·Q ²+θ₁ ·Q+θ₀)·h]ω  (A1)

Where φ₂, φ₁, φ₀, θ₂, θ₁, θ₀, δ₂, δ₁ and δ₀ can be determinedempirically from test data including various combinations of altitudes,motor speed, flow rates, and pressure.

By neglecting the effects of humidity and local air temperature, airdensity can be written as a function of altitude using the values forthe International Standard Atmosphere and the universal gas constant.

The absolute temperature (T) at altitude (h) meters above sea level maybe given by:

T=T ₀ −L·h; and

The pressure (p) at altitude (h) is given by:

${p = {p_{0} \cdot \left( {1 - \frac{L \cdot h}{T_{0}}} \right)^{\frac{g \cdot M}{R \cdot L}}}};$

where:

-   -   T₀ is the sea level standard temperature (e.g. 288.15K);    -   L is the temperature lapse rate (e.g. 0.0065K/m);    -   R is the universal gas constant (e.g. 8.31447 J/(mol*K);    -   M is the molar mass of dry air (e.g. 0.0289644 Kg/mol);    -   p₀ is a sea level standard atmospheric pressure (e.g. 101325        Pa); and    -   g is the earth surface gravitational acceleration (e.g. 9.80665        m/s²).

The air density (ρ)(Kg/m³) can be calculated by the following:

$\rho = \frac{p \cdot M}{R \cdot T}$

-   -   In some cases, this density equation may be simplified by        approximating density as a linear function of altitude. Such a        simplification may be performed to maintain the pressure model        (A1) linear with altitude and to allow calculation of altitude        without the need for expensive computations. The simplified air        density approximation was determined to have a low error rate,        i.e. to be sufficiently accurate, within the range of altitudes        at which a respiratory apparatus is commonly used, such as below        9,000 feet above sea level. A simplified air density        approximation is as follows:

ρ≅α·(1−4·β·h)  (D1)

where:

$\begin{matrix}{{\alpha = \frac{P_{0} \cdot M}{R \cdot T_{0}}};{and}} & ({D2}) \\{\beta = \frac{L}{T_{0}}} & ({D3})\end{matrix}$

-   -   ρ is the air density (Kg/m³);    -   P₀ is a sea level standard atmospheric pressure (e.g., 101325        Pa);    -   M is the molar mass of dry air (e.g., 0.0289644 Kg/mol);    -   R is the universal gas constant (e.g., 8.31447 K/(mol*K);    -   T₀ is the sea level standard temperature (e.g., 288.15K); and    -   L is the temperature lapse rate (e.g. 0.0065K/m).

Making use of the above simplification for air density (A1) canre-written as:

P≅α·(φ₂ ·Q ²+φ₁ ·Q+φ ₀)(1−4·β·h)·ω²+[(δ₂ ·Q ²+δ₁ ·Q+δ ₀)+(θ₂ ·Q ²+θ₁·Q+θ ₀)·h]ω  (A2)

Function ƒ_(x) to compute the altitude of the apparatus 100 may beimplemented without a temperature measure, such as if no temperaturesensor is implemented by the apparatus 100, by solving for h in (A2):

$\begin{matrix}{h \cong \frac{P - \left\lbrack {{{\alpha\left( {{\varphi_{2} \cdot Q^{2}} + {\varphi_{1} \cdot Q} + \varphi_{0}} \right)}\omega^{2}} + {\left( {{\delta_{2} \cdot Q^{2}} + {\delta_{1} \cdot Q} + \delta_{0}} \right) \cdot \omega}} \right\rbrack}{{\left( {{\theta_{2} \cdot Q^{2}} + {\theta_{1} \cdot Q} + \theta_{0}} \right) \cdot \omega} - {4{{\alpha\beta}\left( {{\varphi_{2} \cdot Q^{2}} + {\varphi_{1} \cdot Q} + \varphi_{0}} \right)}\omega^{2}}}} & ({A3})\end{matrix}$

where:

-   -   h is the estimated altitude;    -   P is the measured pressure;    -   Q is the measured flow; and    -   ω is the measured motor speed (e.g., rotational speed).

φ₂, φ₁, φ₀, θ₂, θ₁, θ₀, δ₂, δ₁ and δ₀ may be constants that arepredetermined empirically with a set of test data including variouscombinations of altitudes, motor speed, flow rates, and pressure.

In some embodiments, the function ƒ_(x) to compute the altitude of theapparatus 100 may take into account the temperature. Specifically, themotor speed ω_(temp) under the influence of the temperature maysubstitute for co in equation A1, where ω_(temp) may be a function ofthe measured temperature and the measured motor speed as follows:

ω_(temp)=ω+χ_(n) ·T ^(n)+χ_(n-1) ·T ^(n-4)+χ₁ ·T+χ ₀  (A4)

where:

-   -   ω is the measured motor speed; and    -   Tis the measured temperature.        Here, χ_(n), χ_(n-1), . . . , χ₁, χ₀, along with other constants        φ₂, φ₁, φ₀, θ₂, θ₁, θ₀, δ₂, δ₁ and δ₀ can be determined        empirically from test data including various combinations of        altitudes, motor speed, flow rates, and pressure.

In some embodiments, to increase accuracy or reliability of estimation,the altitude estimation unit 402 may estimate the altitude of theapparatus 100 for a predetermined number of times, e.g., 2, 3, 4, 5, 6,7, 8, 9, 10 times or more, and calculate an average of the estimates.For instance, the altitude estimation unit 402 may take measurementsfrom the pressure sensor 104, flow sensor 106, motor speed sensor 108,and optionally, temperature sensor 110 at a predetermined frequency overa predetermined period of time. The altitude estimation unit 402 maycalculate an estimated altitude based on the measurements at the samefrequency, and calculate an average of the estimated altitudes over thepredetermined period of time. The averaging process may improvereliability of the estimation by reducing the effect of variability ofinput sensor signals, and by reducing artifacts within the signals. Thealtitude estimation unit 402 may provide the average estimation to thealtitude validation unit 404, which, in turn, may determine validity orplausibility of the average estimated altitude.

After calculating the altitude of the apparatus 100, the altitudeestimation unit 402 may output the estimated altitude to the altitudevalidation unit 404. The altitude validation unit 404 may then evaluateaccuracy or plausibility of the estimated altitude. For example, thealtitude validation unit 404 may evaluate the accuracy or plausibilityof the estimated altitude (and thereby evaluate the sensor) by comparingthe estimate to a threshold. An implausible or inaccurate estimatedaltitude, or an average estimated altitude, may be taken as a suggestionthat at least one measurement obtained from one sensor used in computingthe altitude is erroneous. If the flow sensor 106, the motor speedsensor 108, and optionally, the temperature sensor 110 are presumedaccurate, an inaccurate or implausible estimated altitude may suggestthat the pressure measurement obtained from the pressure sensor 104 isincorrect, or the pressure sensor 104 is inaccurate. A slight error inthe pressure sensor 104 may significantly affect the quantity of theestimated altitude.

In one example, the threshold test may involve a range of altitudes inwhich the apparatus 100 may operate, including a maximum altitude and aminimum altitude. In such a case, the apparatus 100 may be implementedwithout an altimeter. Such altitudes may, for example, be in a rangefrom 0 to 9000 feet above sea level or from 500 feet below sea level to10,000 feet above sea level. Other ranges may be used for such arange-based threshold test. When the estimated altitude falls outsidethe range of altitudes, the estimated altitude may be deemedimplausible, which, in turn, may suggest that the pressure sensor 104 isinaccurate. Otherwise, if the estimated altitude is within the range,the pressure sensor 104 may be taken as being sufficiently accurate oroperable for controlling pressure with the apparatus 100.

In another example, the threshold may be an altitude reading obtainedfrom the altimeter 112, in the event that apparatus 100 is provided withan altimeter. The altitude validation unit 404 may evaluate plausibilityof the estimated altitude by comparing it to the altitude reading of thealtimeter 112. When the estimated altitude differs significantly fromthe altitude reading by an offset greater than a predetermineddifference, e.g., 500 feet, 600 feet, 700 feet, 800 feet or 900 feet,the estimated altitude may be deemed inaccurate, which, in turn, maysuggest that the pressure sensor 104 is inaccurate. If the estimatedaltitude does not differ significantly from the altitude reading, thepressure sensor 104 may be taken as being sufficiently accurate oroperable for controlling pressure with the apparatus 100.

In a further example, the threshold may be an altitude previouslycalculated by the altitude estimation unit 402, such as in the case thatno altimeter is implemented. The previously calculated altitude estimatemay have been previously validated by the altitude validation unit 404.The previously calculated altitude estimate may be designated as thecurrent altitude of the apparatus 100. In this regard, the previouslycalculated altitude estimate may have been stored in the memory 118 asresult of a previous validation. In this example, the altitudevalidation unit 404 may receive the estimated altitude output by thealtitude estimation unit 402, and compare it to the current altitude ofthe apparatus 100, e.g., the previously calculated altitude estimate.When the estimated altitude is not the same, or if it differs from thepreviously calculated altitude estimate by an offset greater than apredetermined difference, e.g., 400 feet, 500 feet, 600 feet, 700 feet,800 feet or 900 feet, the estimated altitude may be deemed inaccurate,which, in turn, may be taken as a suggestion that the pressure sensor104 is no longer accurate. Otherwise, if the estimated altitude is thesame as the previously calculated altitude estimate or if it does notdiffer significantly from the previously calculated altitude estimate,the pressure sensor 104 may be taken as still being sufficientlyaccurate or operable for controlling pressure with the apparatus 100.

If the estimated altitude is deemed implausible or inaccurate, thealtitude validation unit 404 may output a signal indicating that thepressure sensor 104 is inaccurate, or a fault has occurred in thepressure sensor 104. The altitude validation unit 404 may output thesignal to the pressure sensor fault handler 122 for further execution.

When the estimated altitude is deemed plausible or accurate, thealtitude validation unit 404 may output a signal indicating that thepressure sensor 104 is accurate, or no fault has occurred in thepressure sensor 104.

With continued reference to FIG. 4 , the detector 120 may optionallyinclude a pressure estimation unit 406 and a pressure validation unit408. The pressure estimation unit 406 may calculate an expected estimateof a pressure. The pressure estimation unit 406 may estimate theexpected pressure from measurements obtained from one or more of thevarious sensors; temperature sensor 110, motor speed sensor 108, andflow sensor 106.

For example, the pressure estimation unit 406 may estimate the expectedpressure from some or all of the following: the measured flow rate ofthe gas obtained from the flow sensor 106, the measured motor speedobtained from the motor speed sensor 108, and the altitude of theapparatus 100, such as a previously calculated altitude or a measuredaltitude from an altimeter 112. The pressure estimation unit 406 maycalculate the expected pressure according to a function:

P _(e)=ƒ_(x)(h Q,T and ω)

where:

-   -   P_(e) is the estimate of expected pressure;    -   h is an estimate, measure or input altitude;    -   Q is a measure of flow;    -   T is an optional measure of temperature that in some embodiments        may be omitted; and    -   ω is a measure of motor speed.

ƒ_(x) is a polynomial function (e.g., y=Ax²+Bx+c of a suitable degreedepending on the constants and parameters of the function) applied tothe measured values that may be derived empirically, such as by a GrayBox Model or best fit model using suitable test data. Such measurementsfor the determination of pressure with such a function may be madeperiodically (e.g., on a measurement cycle of 1, 2, 3, 4, 5, 6, 7, 8, 9or 10 hertz rate) during operation of the apparatus 100, such as duringuse or treatment, and may optionally be performed by the apparatus 100as a start-up or pre-treatment procedure and may take place while theapparatus 100 is not being used by a patient/user (i.e., without thepatient interface 132 on the patient) or both during patient use andpatient non-use operation.

In one example, the function may be implemented without a temperaturemeasure, such as if no temperature sensor is implemented, as follows(from (A2)):

P≅α·(φ₂ ·Q ²+φ₁ ·Q+φ ₀)(1−4·β·h)·ω²+[(δ₂ ·Q ²+δ₁ ·Q+δ ₀)+(θ₂ ·Q ²+θ₁·Q+θ ₀)·h]ω  (B1)

where:

-   -   P_(e) is the expected pressure;    -   Q is the measured flow;    -   ω is the measured motor speed;    -   h is an altitude;    -   α is defined according to equation D2 above; and    -   β is defined according to equation D3 above.

In the above function, φ₂, φ₁, φ₀, θ₂, θ₁, θ₀, δ₂, δ₁ and δ₀ may beconstants that are predetermined empirically from a set of test dataincluding various combinations of altitudes, motor speed, flow rates,and pressure.

In one example, h may be a value obtained from the altimeter 112 ifimplemented. In another example, h may be the altitude of the apparatus100 previously calculated by the altitude estimation unit 402 and storedin the memory 118. The previously calculated altitude may be validatedby the altitude validation unit 404 and designated as the currentaltitude of the apparatus 100.

In another example of the function, such as in the case that a measureof temperature is available, the pressure estimation unit 406 may takeinto consideration the measured temperature of the gas obtained from thetemperature sensor 110. The expected pressure may then be calculatedaccording to a function such as the equation B1 with ω_(temp) beingsubstituted for ω in equation B1. The function ω_(temp) is a function ofmeasured temperature T and measured motor speed ω as described above inequation A3.

In some embodiments, the pressure estimation unit 406 may be configuredto start estimation of the pressure or the timing of the measurement ofvalues for the function based on one or more of the following: flowrate, flow derivative, motor speed and instantaneous acceleration of themotor speed (i.e. rotor acceleration). For example, the measurements forthe application of the pressure estimation unit 406 may be taken at acertain motor speed, a certain flow rate, a certain flow derivative(e.g., approximately zero such as during detection of an end expiratorypause) or certain acceleration of the motor speed (e.g., approximatelyzero).

In one such example, the pressure estimation unit 402 may performestimation of the pressure when the apparatus 100 reaches a steady statecondition, e.g., when the flow generator 102 operates generallyconstantly at a predetermined speed. For instance, the pressureestimation unit 402 may determine the instantaneous acceleration of theflow generator 102. If the instantaneous acceleration is small ornegligible, the motor speed may be deemed steady.

After calculating the expected pressure, the pressure estimation unit406 may output the expected pressure to the pressure validation unit408. The pressure validation unit 408 may validate an actual readingobtained from the pressure sensor 104, i.e., the measured pressure,against the expected pressure.

When the actual reading obtained from the pressure sensor 104 deviatesfrom the expected pressure such as if they are not equal or if theirdifference exceeds a predetermined threshold, e.g., 5 cmH₂O, thepressure sensor 104 may be deemed inaccurate (e.g., if(|P_(meas)−P_(est)|)>5 cmH₂O). The pressure validation unit 408 mayoutput a signal indicating that the pressure sensor 104 is inaccurate tothe pressure sensor fault handler 122 for further execution.

However, if the difference between the measured pressure and theexpected pressure is equal to or less than the predetermined threshold,the pressure sensor 104 may be deemed accurate (e.g., if((|P_(meas)−P_(est)|) 5 cmH₂O). The pressure validation unit 408 mayoutput a signal indicating that the pressure sensor 104 is accurate, orno fault has occurred in the pressure sensor 104.

It is to be understood that the predetermined threshold may includeother limits such as 1 cm H₂O, 2 cmH₂O, 3 cmH₂O, 4 cmH₂O, or 6 cmH₂O orsimilar such limits. In some cases, the predetermined threshold mayinclude limits determined from relative values rather than absolutevalues. For instance, the threshold may be a function of the expected ormeasured pressure. For example, the predetermined threshold may bedetermined using a portion or percentage of the expected pressure. Otherthresholds described in this specification may be similarly derived asfunctions of other values.

In an example embodiment, the detector 120 may execute an initial testof the pressure sensor 104 prior to therapeutic treatment to thepatient, such as part of an initialization procedure. Thereafter, thedetector 120 may execute one or more periodic tests of the pressuresensor 104 after therapeutic treatment commences. Detailedimplementations of various test procedures performed by detector 120 arediscussed below with reference to FIGS. 5-12 . Unless otherwiseindicated, any of the procedures described below may be performed beforeor during therapeutic treatment to the patient. Further, unlessotherwise indicated, the procedures described below may be performed inan initial test, one or more of periodic tests, or the combinationthereof.

FIG. 5 is a flowchart 500 illustrating a test procedure that may beperformed by the detector 120 as an initialization procedure for theapparatus 100. At 502, the detector 120 may set a predetermined flowrate for delivery by a blower 130 of the flow generator 102. Forexample, the predetermined flow rate may be a flow rate of 20liters/minute, or a low flow rate that is below 50 liters/minute.Alternatively, the predetermined flow rate may be in a range betweenabout 10 liters/minute and about 60 liters/minute. In one embodiment,the predetermined flow rate may be manually entered by a user atinitialization of the apparatus 100, e.g., when the apparatus 100 ispowered on. In another embodiment, the predetermined flow rate may be apre-set parameter for the methodology of the initialization proceduresuch as a value recorded in the memory 118.

Thereafter, between 504 and 508, the detector 120 may control the flowgenerator 102 to adjust the motor speed until the measured flow rateobtained from the flow sensor 106 meets the predetermined flow rate. Inone embodiment, after the apparatus 100 powers on, the flow generator102 may gradually increase the motor speed. Specifically, at step 504,the flow generator 102 may increase the motor speed in a predeterminedincrement at a predetermined frequency. After each increment, at step506, the detector 120 may take a reading from the flow sensor 106 whichrepresents the measured flow rate. At step 508, the detector 120 maycompare the measured flow rate to the predetermined flow rate. If themeasured flow rate is below the predetermined flow rate, the detector120 may proceed back to step 504, at which point, the flow generator 102may continuously increase the motor speed.

Once the measured flow rate reaches the predetermined flow rate, thealtitude estimation unit 402 of the detector 120 may proceed to the nextstep 510.

Steps 502-508 are one example for controlling the blower 130 by using aflow control loop so as to operate the blower 130 at a constant or fixedflow rate. Alternatively, as previously mentioned, the blower 130 may becontrolled via a motor speed control loop so as to operate the blower130 to maintain a constant/fixed motor speed rather than a constant orfixed flow rate.

At 510-512, the detector 120 may make measurements from various sensorsincluding: the pressure sensor 104, the motor speed sensor 108, and,optionally, the temperature sensor 110. Thereafter, at 514, the altitudeestimation unit 402 may determine or calculate the altitude of theapparatus 100 from the measurements using a suitable function such asone based on function A1 or a look-up table with data derived from sucha function.

Subsequently, at 516, the altitude validation unit 404 may evaluate theplausibility of the altitude calculated at 514. In one example, thealtitude validation unit 404 may evaluate the calculated altitude bycomparing the altitude calculated at 514 to a predetermined range ofexpected altitudes, examples of which are discussed earlier. In such acase, the apparatus 100 may be implemented without an altimeter. If thecalculated altitude falls outside the predetermined range, thecalculated altitude may be deemed implausible or inaccurate. Otherwise,if the calculated altitude falls within the predetermined range ofexpected altitudes, the calculated altitude may be deemed plausible andthe pressure sensor may be deemed acceptable for normal operation of theapparatus 100.

Alternatively, the altitude validation unit 404 may evaluate theplausibility of the calculated altitude by comparing the calculatedaltitude to a reading of the altimeter 112 if implemented in theapparatus 100. The reading of the altimeter 112 may represent anaccurate measurement of the altitude of the apparatus 100. If thealtitude calculated at 514 differs from the measured altitude by morethan a predetermined amount, e.g., 600 feet or 900 feet, then thecalculated altitude may be taken as inaccurate and the pressure sensormay be deemed not suitable for normal operation. Otherwise, thecalculated altitude may be taken as acceptable and the pressure sensormay then be deemed suitable for normal operation.

In another embodiment, the altitude validation unit 404 may evaluate theplausibility of the altitude calculated at 514 by comparing it to apreviously determined altitude, such as one determined from a previoussession with the apparatus 100. The previously determined altitude maybe retrieved from the memory 118. The previously determined altitude maybe calculated by the altitude estimation unit 402 in a previous testprocedure. The previously determined altitude may have been validated bythe altitude validation 404 in the previous test. The previouslydetermined altitude may be taken to represent the current altitude ofthe apparatus 100. If the altitude calculated at 514 differs from thepreviously determined altitude by a predetermined amount or threshold,e.g., 400 feet, 500 feet, 600 feet, 700 feet, 800 feet or 900 feet, thenthe presently calculated altitude may be taken as inaccurate, and thepressure sensor may be deemed unsuitable for normal operation of theapparatus 100. Otherwise, the calculated altitude may be taken asacceptable, and the pressure sensor 104 may be deemed suitable foroperation.

Once the calculated altitude is deemed implausible or unacceptable, thenat 522, the altitude validation unit 404 may output a signal indicatingthat the pressure sensor 104 is inaccurate or not suitable for normaloperation. By contrast, if the calculated altitude is deemed plausibleor acceptable, then at 518, the altitude validation unit 404 may outputa signal indicating that the pressure sensor 104 is accurate or suitablefor normal operation. When the calculated altitude is deemed plausible,the detector 120 may optionally, at step 520, store the calculatedaltitude in the memory 118. The calculated altitude may be subsequentlyretrieved from the memory 118 during one or more periodic testprocedures.

In the example flow chart of the test procedure illustrated in FIG. 5 ,the process of the apparatus 100 may evaluate the pressure sensor 104using one cycle of analysis, which may include one instance of each ofthe following: taking a pressure measurement using the pressure sensor104, taking a motor speed measurement using the motor speed sensor 108,optionally taking a temperature measurement using the temperature sensor110, and calculating or determining the altitude based on thesemeasurements and evaluating the calculated altitude.

FIG. 6 illustrates an alternate flow chart 600 embodiment to the testprocedure illustrated in FIG. 5 . In this embodiment, the detector 120may take a fault-tolerant approach to assess accuracy of the pressuresensor 104 such as by utilizing averages. For instance, the detector 120may determine accuracy of the pressure sensor 104 based on a collectiveresult of multiple cycles of analysis. Each cycle of analysis may beidentical to that described above with respect to FIG. 5 . Each cycle ofanalysis may occur at a predetermined frequency. For example, thepredetermined frequency may be between 1 and 10 hertz, such as 1, 2, 3,4, 5, 6, 7, 8, 9 or 10 hertz sampling rate. The collective result may bean average calculated altitude obtained from multiple cycles of analysisthat occur within a predetermined period of time. The predeterminedperiod of time may be, for example, 5, 10, 15, 20 or 30 seconds orlonger or shorter periods of time. In the illustrated example, thedetector 120 may include a timer to trigger multiple cycles for analysisor otherwise may implement a repeated measurement loop. The pressuresensor 104 may be determined suitable for normal operation or not whenan average of calculated altitudes, or a calculated altitude fromaveraged measurements, determined from multiple cycles is evaluated aspreviously described.

As shown in FIG. 6 , the processes of 602-608 may be identical to502-508 of FIG. 5 . At 610, the detector 120 may start a timer for themultiple cycles of analysis. Subsequently, the altitude estimation unit402 may perform at 612-616 in the manner described with reference to510-514 of FIG. 5 . An iteration of 612-616 may be considered one cycleof analysis. The detector 120 may repeatedly iterate 612-616 to performmultiple cycles of analysis, until a predetermined amount of time, e.g.,5 seconds, has elapsed since the start of the timer as determined at618.

Thereafter, at 620, the altitude estimation unit 402 may calculate anaverage of the altitudes calculated during the multiple cycles ofanalysis. The altitude estimation unit 402 may output the average to thealtitude validation unit 404.

Then, at 622-628, the altitude validation unit 404 may determinesuitability of the pressure sensor 104 based on plausibility of theaverage altitude as previously described with reference to 516-522 ofFIG. 5 .

In another embodiment, the timer 610 may be replaced with a counterconfigured to count the number of cycles and step 618 may determine ifthe desired number of cycles have been performed. In such an arrangementthe detector 120 may assess accuracy of the pressure sensor 104 based ona predetermined number of cycles of analysis, e.g., at least 2 cycles ofanalysis or 5-10 cycles of analysis or more. The predetermined number ofcycles may or may not be consecutive. For example, the detector 120 mayconclude that the pressure sensor 104 is not suitable for normaloperation when the altitude calculated is deemed implausible in each of5-10 consecutive cycles of analysis. Alternatively, the detector 120 mayconclude that the pressure sensor 104 is inaccurate when an average ofthe altitudes calculated in the 5-10 cycles of analysis is determinedimplausible.

Subsequent to an initialization procedure, the detector 120 mayperiodically (e.g., continuously) monitor the accuracy of the pressuresensor 104 in one or more test procedures as the apparatus 100 providestherapeutic treatment to the patient. FIG. 7 illustrates a flowchart 700of a periodic test procedure according to one such embodiment. At 702,the detector 120 or the altitude validation unit 404 may retrieve apredetermined or previously determined altitude from the memory 118. Thepredetermined altitude may be calculated by the altitude estimation unit402 during a previous test, e.g., an initialization procedure test,which had been previously stored in the memory 118. The previouslydetermined altitude may represent an estimation of the altitude of theapparatus 100 that has been deemed plausible or acceptable.Alternatively, the predetermined altitude may be a reading obtained fromthe altimeter 112. The reading may also represent an accurate estimationof the altitude of the apparatus 100.

With continued reference to FIG. 7 , the process at 704-708 may beidentical to the process at 510-514 of FIG. 5 , in which the altitudeestimation unit 402 reads measurements from the pressure sensor 104, themotor speed sensor 108, and, optionally, the temperature sensor 110, andcalculates the altitude of the apparatus 100 based on these measurementsand the functions or equations previously described.

Thereafter, at 710, the altitude validation unit 404 may compare thealtitude calculated by the altitude estimation unit 402 at 708 to thepredetermined altitude obtained at step 702. If the altitude calculatedat 708 reasonably approximates the previously determined altitude of theapparatus 100, then the measurements based on which the altitude iscalculated may be deemed correct, which may suggest that the pressuresensor 104 is accurate or continues to be suitable for normal operationof the apparatus 100. For instance, if the difference between thecalculated altitude and the predetermined altitude does not exceed apredetermined amount or threshold, e.g., 400 feet, 500 feet, 600 feet,700 feet, 800 feet or 900 feet, then the pressure sensor 104 may bedeemed suitable or accurate. As a result, the altitude validation unit404 may, at 714, output a signal indicating that the pressure sensor 104is accurate.

However, if the altitude calculated at 708 does not reasonablyapproximate the previously determined altitude of the apparatus 100,then the pressure sensor 104 may be deemed faulty. For instance, if thedifference between the calculated altitude and the predeterminedaltitude exceeds a predetermined threshold, e.g., 400 feet, 500 feet,600 feet, 700 feet, 800 feet or 900 feet, the pressure sensor 104 may bedeemed inaccurate. The altitude validation unit 404 may, at 716, outputa signal indicating that the pressure sensor 104 is inaccurate.

The periodic test procedure illustrated in FIG. 7 may assess thepressure sensor 104 using one cycle of analysis, which includes oneinstance of each of the following: taking a pressure measurement usingthe pressure sensor 104, taking a motor speed measurement using themotor speed sensor 108, optionally taking a temperature measurementusing the temperature sensor 110, and calculating and evaluating thealtitude based on these measurements. Such a periodic test procedure mayoptionally be implemented on a frequency of 1-10 hertz, such as 1, 2, 3,4, 5, 6, 7, 8, 9 or 10 hertz sampling rate.

FIG. 8 illustrates an alternate flow chart 800 embodiment to theperiodic test procedure illustrated in FIG. 7 . In this embodiment, thedetector 120 may take a fault-tolerant approach to assess accuracy ofthe pressure sensor 104 based on averages. For instance, the detector120 may determine accuracy of the pressure sensor 104 based on acollective result or average of multiple cycles of analysis. Each cycleof analysis may occur at a predetermined frequency, e.g., between 1 and10 hertz, such as 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 hertz sampling rate.

As shown in FIG. 8 , the processing at 802 may be identical to theprocessing at 702. At 804, the detector 120 may start a timer for themultiple cycles of analysis. Subsequently, the altitude estimation unit402 may perform the processing at 806-810, which may be identical to theprocessing at 704-708 of FIG. 7 . An iteration of the processing at806-810 may be considered as one cycle of analysis. The altitudeestimation unit 402 may repeatedly iterate the processing at 806-810 toperform multiple cycles of analysis, until a predetermined amount oftime, e.g., 5, 10, 15, 20 or 30 seconds or longer or shorter periods oftime, has elapsed since the start of the timer as determined at 812.

Thereafter, at 814, the altitude estimation unit 402 may calculate anaverage of the altitudes calculated during the multiple cycles ofanalysis or an altitude of average measurements from the multiplecycles. The altitude estimation unit 402 may output the average to thealtitude validation unit 404.

Then, at 816-822, the altitude validation unit 404 may determineaccuracy of the pressure sensor 104 based on analysis of the averagealtitude. In this regard, the processing at 816-822 may be identical tothe processing at 710-716 of FIG. 7 .

In a further embodiment, the timer 804 may be replaced with a counterconfigured to count the number of cycles and step 812 may determine ifthe desired number of cycles have been performed. In such an arrangementthe detector 120 may assess accuracy of the pressure sensor 104 based ona predetermined number of cycles of analysis, e.g., at least 2 cycles ofanalysis or 5-10 cycles of analysis or more. The predetermined number ofcycles may or may not be consecutive. For instance, the detector 120 mayconclude that the pressure sensor 104 is inaccurate when the differencebetween the calculated altitude and the previously determined altitudeexceeds the threshold amount in each of the at least 2 or 5-10consecutive cycles of analysis. Alternatively, the detector 120 mayconcluded that the pressure sensor 104 is inaccurate when an averagealtitude calculated from the at least 2 or 5-10 cycles of analysisdiffers from the previously determined altitude by an offset greaterthan the predetermined threshold.

FIG. 9 includes a flowchart 900 of a periodic test procedure accordingto another embodiment. At 902, the detector 120 may retrieve apreviously determined altitude of the apparatus 100. Thus, theprocessing at 902 may be identical to the processing at 702 or 802 ofFIGS. 7 and 8 respectively. At 904, the detector 120 or the pressureestimation unit 406 may obtain measurements from the flow sensor 106,the motor speed sensor 108, and, optionally, the temperature sensor 110.Thereafter, at 906, the pressure estimation unit 406 may calculate anexpected pressure based on the predetermined altitude and the flow,motor speed and/or temperature measurements at 904 such as by thefunction previously mentioned (e.g., function B1 or a look-up tablebased thereon).

Then, at 908-910, the pressure validation unit 408 may obtain pressuremeasurement from the pressure sensor 104, and compare the measuredpressure to the expected pressure. At 912, if the difference between themeasured pressure and the expected pressure is equal to or less than apredetermined threshold, e.g., 5 cmH₂O, the pressure sensor 104 may bedeemed accurate. Then, at 914, the pressure validation unit 408 mayoutput a signal indicating that the pressure sensor 104 is accurate. Asnoted above the predetermined threshold may include other limits such as1 cm H₂O, 2 cmH₂O, 3 cmH₂O, 4 cmH₂O, or 6 cmH₂O or similar such limits.

By contrast, if the difference between the measured pressure and theexpected pressure exceeds the predetermined threshold, the pressuresensor 104 may be deemed inaccurate. Then, at 916, the pressurevalidation unit 408 may output a signal indicating that the pressuresensor 104 is not suitable for normal operation.

FIG. 10 illustrates an alternate flow chart 1000 embodiment to the testprocedure illustrated in FIG. 9 . In this embodiment, the detector 120may take a fault-tolerant approach to assess accuracy of the pressuresensor 104 based on multiple cycles of analysis over a predeterminedperiod of time. The predetermined period of time may be, for example, 5,10, 15, 20 or 30 seconds or longer or shorter periods of time. Thedetector 120 may include a timer or other iterative procedure toimplement the repeated cycles of analysis. Each cycle of analysis mayoccur at a predetermined frequency, e.g., between 1 and 10 hertz, suchas 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 hertz. Each cycle of analysis mayinclude measuring flow, motor speed, and optionally, temperature,calculating expected pressure based on these measurements, comparing areading of the pressure sensor 104 to the expected pressure. Thepressure sensor 104 may be determined inaccurate or not suitable fornormal operation when each pressure measurement differs from theexpected pressure by an offset exceeding a threshold amount, e.g., 5cmH₂O, over a predetermined period of time.

As shown in FIG. 10 , the processing at 1002-1014 may be identical tothe processing at 902-914 of FIG. 9 . An iteration of the processing at1004-1012 may be considered as one cycle of analysis. The detector 120may repeatedly iterate steps 1004-1012 to perform multiple cycles ofanalysis. If the measured pressure differs from the expected pressure byan offset greater than a threshold amount, e.g., 5 cmH₂O, at 1012, thedetector 120 may proceed to 1016. At 1016, the detector 120 maydetermine whether a timer for the multiple cycles of analysis hasstarted. If the timer has not started, the detector 120 may start thetimer at 1018, and may then proceed to 1004 to start a new cycle ofanalysis. If the timer has already started, then the detector 120 may,at 1020, determine whether the predetermined amount of time, e.g., 5,10, 15, 20 or 30 seconds, has elapsed since the start of the timer. Ifnot, the detector 120 may proceed to 1004 to start a new cycle ofanalysis. If yes, the detector 120 may proceed to 1022 to output asignal indicating that the pressure sensor 104 is inaccurate. At thatpoint, the detector 120 may optionally stop or reset the timer. As notedabove the predetermined threshold may include other limits such as 1 cmH₂O, 2 cmH₂O, 3 cmH₂O, 4 cmH₂O, or 6 cmH₂O or similar such limits.

If during any cycle of analysis, the difference between the measuredpressure and the expected pressure does not exceed the predeterminedthreshold at 1012, the detector 120 may, at 1014, output a signalindicating that the pressure sensor 104 is accurate or suitable fornormal operation.

In a further embodiment, the timer 1018 may be replaced with a counterconfigured to count the number of cycles and step 1020 may determine ifthe desired number of cycles have been performed. In such an arrangementthe detector 120 may assess accuracy of the pressure sensor 104 based ona predetermined number of cycles of analysis, e.g., 10 cycles ofanalysis. The predetermined number of cycles may or may not beconsecutive. For instance, the detector 120 may conclude that thepressure sensor 104 is inaccurate when the difference between thecalculated altitude and the predetermined altitude exceeds the thresholdamount (e.g., ((|P_(meas)−P_(est)|)>5 cmH₂O)) in each of the 10consecutive cycles of analysis.

In some embodiments, the detector 120 may perform an initial test of thepressure sensor 104 according to the steps described in FIG. 5 or FIG. 6before the apparatus 100 provides therapeutic treatment to a patient.The detector 120 may continuously monitor the accuracy of the pressuresensor 104 in one or more periodic tests as the apparatus 100 providestherapeutic treatment to the patient according to procedures describedin any one of FIGS. 7-10 . In one embodiment, the predetermined altituderetrieved at one of the steps 702, 802, 902 and 1002 may be an altitudecalculated by the altitude estimation unit 402 in the initial test. Inanother embodiment, the predetermined altitude may be a reading obtainedfrom the altimeter 122. Alternatively, the predetermined altitude may bea range of expected altitudes, examples of which are describedpreviously.

In another embodiment, an initial test may be implemented usingprocedures described in any one of FIGS. 7-10 . In this embodiment, thepredetermined altitude retrieved at any one of the processes 702, 802,902 and 1002 may be an altitude calculated by the altitude estimationunit 402 in a previous test related to a previous therapeutic treatment.Alternatively, the predetermined altitude may be a reading obtained fromthe altimeter 122. The predetermined altitude may also represent a rangeof expected altitudes that the apparatus 100 may operate.

In a further embodiment, the detector 120 may constantly test theaccuracy of the pressure sensor 104 by executing procedures described inany one of FIGS. 5-10 at a predetermined interval before and throughoutthe therapeutic treatment to the patient.

Dual Pressure Sensors

While the examples previously described herein permit evaluation of ameasurement of a pressure sensor without implementing a second pressuresensor and in some cases without an altimeter, in some alternativeembodiments, measurements of a second or back-up pressure sensor may beimplemented to check the pressure of a first pressure sensor. Forexample, the apparatus 100 may optionally include a second pressuresensor 124 as illustrated in FIG. 1 . The second pressure sensor 124 maybe positioned in a close proximity to the first pressure sensor 104,such that both pressure sensors 104, 124 may measure pressure of the gasat the same location. For example, both pressure sensors 104, 124 may bepositioned at the patient interface 132 or at the blower 130. If thefirst pressure measurement differs from the second pressure measurementby an offset greater than a predetermined threshold, e.g., 5 cmH₂O, thefirst pressure sensor 104 may be deemed inaccurate. Otherwise, the firstpressure sensor 104 may be deemed accurate.

Alternatively the second pressure sensor 1124 may be positioned at adifferent location to the first pressure sensor 104, such as the firstpressure sensor 104 may be positioned at the patient interface 132 andthe second pressure sensor 124 positioned at the blower 130 or viceversa. The pressure drop between the two pressure sensors may be known,estimated or characterized such that the pressure drop between thepressure sensors is taken into consideration when comparing the pressuremeasurements obtained from the first pressure sensor 104 and the secondpressure sensor 124. Thus the difference between the pressuremeasurements from first pressure sensor 104 and the second pressuresensor 124 may not exceed a predetermined threshold that is determinedas a function of the pressure drop between the first pressure sensor 104and the second pressure sensor 124.

FIG. 11 illustrates a flowchart 1100 of a method performed by thedetector 120 according to the above embodiment. At 1102, the detector120 may obtain a first pressure measurement from the first pressuresensor 104. At 1104, the detector 120 may obtain a second pressuremeasurement from the second pressure sensor 124. At 1106, the detector120 may compare the first pressure measurement to the second pressuremeasurement. Thereafter, at 1108, the detector 120 may determine if thefirst pressure measurement differs from the second pressure measurementby an offset greater than a threshold amount, e.g., 5 cmH₂O. If no, thedetector 120 may at 1110 issue a signal indicating that the firstpressure sensor 104 is accurate. If yes, the detector 120 may, at 1112,issue a signal indicating that the first pressure sensor 104 isinaccurate.

As noted above the predetermined threshold may include other limits suchas 1 cm H₂O, 2 cmH₂O, 3 cmH₂O, 4 cmH₂O, or 6 cmH₂O or similar suchlimits.

Response to Faulty Sensor Detection

If the detector 120 determines that the pressure sensor 104 isinaccurate or not suitable for normal operation, then a fault has beendetected in the pressure sensor 104.

In the absence of such a fault, the apparatus 100 may operate normally(e.g., with a pressure control loop that relies on the pressuremeasurements from the pressure sensor 104 for control of the blower 130and the patient's treatment).

In the detection of a fault, the apparatus 100 may respond with anautomatic shutdown to prevent further execution. Alternatively, theapparatus 100 may record data of the fault and/or issue a warning or afault message to the user via the user I/O devices 114. For instance, adisplay (not shown) may display a message to the user that the pressuresensor 104 is inaccurate and needs to be replaced. In such a case, thecontroller, e.g., the processor 116, may be configured to preventoperation of the blower 130.

In some embodiments, the apparatus 100 may include a pressure sensorfault handler 122 to control the apparatus 100, once a fault has beendetected in the pressure sensor 104. In these embodiments, the apparatus100 may optionally be permitted to operate in a safe mode, despite thedetection of a fault. For example, the handler 122 may include twoalternate operational safe modes: a pressure-controlled, speed limitedmode or a speed-controlled mode, in which the apparatus 100 may operate.The handler 122 may enter either one of the operational modes once thepressure sensor 104 is determined to not be suitable for normaloperation.

In the pressure-controlled, speed limited mode, the handler 122 mayadjust the motor speed of the flow generator 102 by relying on thepressure measurement by the pressure sensor 104 (e.g., controlling theblower 130 using a pressure control loop), irrespective of the fact thatpressure sensor 104 is deemed inaccurate. More specifically, the handler122 may increase the motor speed until the measured pressure from thepressure sensor 104 reaches a desired pressure level, but in no event,may the motor speed exceed a predetermined maximum motor speed. In sucha case, a maximum motor speed limit will act to override a pressuredemand from the pressure control loop to prevent further increase in themotor speed beyond the limit despite a pressure set point falling shortof the measured pressure from the pressure sensor 104.

FIG. 12 illustrates a flowchart 1200 describing operations of thehandler 122 in the pressure-controlled mode according to one embodiment.At 1202, the handler 122 may determine a desired pressure level settingfor the gas to be achieved with the flow generator 102. The desiredpressure level may be the desired therapeutic treatment pressure to beapplied to the patient. In one example, the desired pressure level maybe manually entered by the user via the user I/O devices 114. In anotherexample, the processor 114 or the handler 122 may calculate the desiredpressure level based on the condition of the patient. The desiredpressure level may be a constant pressure value expected to bemaintained throughout one therapeutic treatment session. Alternatively,the desired pressure level may vary throughout one therapeutic treatmentsession. For instance, during one therapeutic treatment session, thedesired pressure level during inhalation may be greater than the desiredpressure level during exhalation. Additionally, the desired pressurelevel may be automatically adjusted and determined based on detectedbreathing conditions (e.g., obstructive apnea, obstructive hypopnea,flow limitation, snoring, other sleep disordered breathing events,etc.).

At 1204, the handler 122 may set a motor speed threshold. The motorspeed threshold may represent a maximum motor speed that the flowgenerator 102 is allowed to attain, above which the flow generator 102or other components of the apparatus 100 may deteriorate or malfunctionor may be deemed dangerous for a patient. In one example, the handler122 may set the motor speed threshold to an RPM threshold, such as20,000 RPM, 30,000, RPM, 50,000 RPM, 100,000 RPM, 200,000 RPM or othersuitable speeds. The RPM threshold may be determined based on anapproximate expected motor speed required to meet the desired pressureor a maximum pressure from previous system characterization, and may,for example, be obtained from a look-up table of the memory of theapparatus.

At 1206, the handler 122 may obtain a pressure reading from the pressuresensor 104. At 1208, the handler 122 may compare the measured pressureto the desired pressure level. If the measured pressure reaches thedesired pressure level, the handler 122 may terminate the adjustmentoperation. Otherwise, the handler 122 may, at 1210, change the nextmotor speed for the flow generator 102. In one example, the next motorspeed may be increased from the present motor speed by a predeterminedincrement. Thereafter, at 1212, the handler 122 may determine if thenext motor speed exceeds the motor speed threshold. If yes, the handler122 may terminate the adjustment operation. If not, the handler 122 may,at 1214, instruct the flow generator 102 to change from its presentmotor speed to the next motor speed. Thereafter, the handler 122 mayproceed to 1206 to measure pressure using the pressure sensor 104.

In the speed-controlled mode, the apparatus 100 may no longer operate bya pressure control loop but may operate in a speed control loop. In sucha mode, the handler 122 may determine a desired motor speed of the flowgenerator 102 that correlates to the desired pressure level (such aswith a look-up table or suitable function to relate pressure to a motorspeed setting), such that when the flow generator 102 operates at thedesired motor speed, the gas generated by the flow generator 102 may bedeemed to be at the desired pressure level. Thus, in thespeed-controlled mode, the handler 122 may determine the desired motorspeed without taking into consideration the pressure measurementobtained from the pressure sensor 104. In one example, the handler 122may include a look-up table that defines a correlation between aplurality of different motor speeds and a plurality of differentpressure levels. The look-up table may define one-on-one mappingsbetween the plurality of different motor speeds and the plurality ofdifferent pressure levels. Once a desired pressure level is determined,the handler 122 may look up its corresponding motor speed, i.e., thedesired motor speed, in the look-up table. The handler 122 may then setthe flow generator 102 to the desired motor speed.

FIG. 13 illustrates a flowchart 1300 describing operations of thehandler 122 in the speed-controlled mode according to one embodiment. At1302, the handler 122 may determine a desired pressure level to be set.The processing at 1302 may be identical to the processing at 1202. At1304, the handler 122 may look up a desired motor speed correlated tothe desired pressure level in the look-up table. At 1306, the handler122 may servo-control the flow generator 102 to the desired motor speedin a speed control loop.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent technology. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present technology.

For example, although versions of the technology described herein havegenerally referred to a determination, estimation or measurement ofaltitude in its methodologies such that the altitude (or elevation) isserving as a subordinate or secondary characteristic of the respiratorytreatment apparatus that may be derived from pressure and/or from whichpressure is derivable by some calculable function(s) that may involvesome measured system variable(s), it will be understood that someversions of the present technology may alternatively implementcomponents that make a determination, estimation or measurement of someother such subordinate or secondary characteristic rather than altitudein any of the previously described methodologies.

For example, in some such versions, the detector 120 may obtain apressure measure from the pressure sensor 104. The detector 120 mayevaluate accuracy of the pressure sensor 104 based on the pressuremeasure and a subordinate or secondary characteristic, such asatmospheric pressure or the atmospheric pressure external to theapparatus 100. Such atmospheric pressure of the apparatus 100 mayinclude one or more of (1) a previously calculated atmospheric pressureestimate from a prior pressure measure of the apparatus 100 (e.g., apressure measure in the flow path of the apparatus which generating apressure above atmospheric pressure), (2) an expected atmosphericpressure or atmospheric pressure range for the apparatus 100, (3) anactual or expected atmospheric pressure input into the apparatus 100 foroperation; and/or (4) an atmospheric pressure measured by an externalsensor of the apparatus 100 such as a barometer. The detector 120 maycalculate an atmospheric pressure estimate based on the pressuremeasure, and compare the atmospheric pressure estimate to any one ormore of the atmospheric pressure (1)-(4) described above. Still othersecondary characteristics may also be implemented in any of themethodologies described herein.

For example, in the case of the subordinate characteristic beingatmospheric pressure, the altitude estimation unit 402 may instead be anatmospheric pressure estimation unit. Thus it may calculate an estimateof the atmospheric pressure during operation of the apparatus 100 as afunction of some or all of the following: the measured pressure of thegas obtained from the pressure sensor 104, the measured flow rate of thegas obtained the flow sensor 106, the measured temperature obtained fromthe temperature sensor 110, and the measured motor speed obtained fromthe motor speed sensor 108. For example, an atmospheric pressureestimation unit may compute the atmospheric pressure of the apparatus100 according to the following function:

Atm=ƒ_(x)(P,Q,T and ω)

where:

-   -   Atm is the estimated atmospheric pressure;    -   P is the measure of pressure;    -   Q is a measure of flow;    -   T is an optional measure of temperature that in some embodiments        may be omitted; and    -   ω is a measure of motor speed.

ƒ_(x) may be a polynomial function (e.g., y=Ax²+Bx+c of a suitabledegree depending on the constants and parameters of the function)applied to the measured values that may be derived empirically, such asby a Gray Box Model or best fit model using suitable test data. Othersuch functions may be implemented similar to those previously describedwith reference to altitude. Similarly, the altitude validation unit 404may be implemented as an atmospheric pressure validation unit.

ADDITIONAL TECHNOLOGY EXAMPLES

Example 1. A method for determining accuracy of a pressure sensor in arespiratory device, comprising:

-   -   measuring pressure of a flow of breathable gas generated by the        respiratory device using the pressure sensor; and    -   determining, with a processor, accuracy of the pressure sensor        based on the measured pressure and a subordinate characteristic        of the respiratory device.

Example 2. The method of Example 1, wherein the respiratory deviceincludes a flow generator with a motor included therein to generate theflow of gas.

Example 3. The method of any one of Examples 1-2, wherein thesubordinate characteristic of the respiratory device is input by a user.

Example 4. The method of any one of Examples 1-2, wherein thesubordinate characteristic of the respiratory device is measured by analtimeter or barometer of the respiratory device.

Example 5. The method of any one of Examples 1-4, wherein the processorcalculates the subordinate characteristic of the respiratory device.

Example 6. The method of Example 5, wherein the processor determines theaccuracy of the pressure sensor based on an evaluation of the calculatedsubordinate characteristic.

Example 7. The method of any one of Examples 5-6, wherein the processorcalculates the subordinate characteristic of the respiratory device as afunction of (a) the pressure measured by the pressure sensor and one orboth of (b)(1) a measured flow rate of the flow of breathable gas and(b)(2) a measured motor speed of the flow generator.

Example 8. The method of Example 7, wherein the processor calculates thesubordinate characteristic of the respiratory device as a function of ameasured temperature of the flow of breathable gas.

Example 9. The method of any one of Example 5-8, wherein the processorcalculates the subordinate characteristic of the respiratory device whenthe flow generator controls the gas at a constant predetermined flowrate.

Example 10. The method of Example 9, wherein the constant predeterminedflow rate is 20 liters/minute.

Example 11. The method of Example 9, wherein the constant predeterminedflow rate is less than 50 liters/minute.

Example 12. The method of Example 9, wherein the constant predeterminedflow rate is in the range between about 10 liters/minute and about 60liters/minute.

Example 13. The method of any one of Examples 5-8, wherein thesubordinate characteristic of the respiratory device is calculated whenthe flow generator controls a constant predetermined motor speed.

Example 14. The method of any one of Examples 6-13, wherein theprocessor evaluates the calculated subordinate characteristic bycomparing the calculated subordinate characteristic with a predeterminedrange of the subordinate characteristic.

Example 15. The method of Example 14, wherein the predetermined range ofthe subordinate characteristic corresponds to a range between 0 and 9000feet above sea level.

Example 16. The method of Example 14, wherein the predetermined range ofthe subordinate characteristic corresponds to a range between 500 feetbelow sea level and 10,000 feet above sea level.

Example 17. The method of any one of Examples 14-16, wherein theprocessor deems the calculated subordinate characteristic acceptablewhen the calculated subordinate characteristic is within thepredetermined range of the subordinate characteristic.

Example 18. The method of any one of Examples 14-17, wherein theprocessor deems the calculated subordinate characteristic unacceptablewhen the calculated subordinate characteristic is outside thepredetermined range of the subordinate characteristic.

Example 19. The method of any one of Examples 5-18, wherein theprocessor calculates the subordinate characteristic of the respiratorydevice at a predetermined frequency over a predetermined period of time.

Example 20. The method of Example 19, wherein the predeterminedfrequency is between 1 and 2 hertz.

Example 21. The method of any one of Examples 19-20, wherein thepredetermined period of time is 5 seconds.

Example 22. The method of any one of Examples 19-21, wherein theprocessor evaluates the pressure sensor based on an average of thecalculated subordinate characteristic.

Example 23. The method of any one of Examples 19-22, wherein theprocessor determines the pressure sensor accurate when an average of thecalculated subordinate characteristic satisfies a threshold comparison.

Example 24. The method of any one of the preceding Examples, wherein theprocessor evaluates the pressure sensor in an initialization processbefore the respiratory device provides treatment to a patient.

Example 25. The method of any one of the preceding Examples, furthercomprising storing the subordinate characteristic of the respiratorydevice in a memory.

Example 26. The method of any one of Examples 2-25, wherein theprocessor evaluates accuracy of the pressure sensor by:

-   -   calculating expected pressure of the gas generated by the        respiratory device; and    -   comparing the measured pressure with the expected pressure.

Example 27. The method of Example 26, wherein the processor calculatesthe expected pressure from the subordinate characteristic of therespiratory device, a measured flow rate of the flow of breathable gas,and a measured motor speed of the flow generator.

Example 28. The method of Example 27, wherein the processor calculatesthe expected pressure from a measured temperature of the flow ofbreathable gas.

Example 29. The method of any one of Examples 26-28, wherein theprocessor determines the accuracy of the pressure sensor by comparing adifference between the measured pressure and the expected pressure witha predetermined threshold.

Example 30. The method of Example 29, wherein the predeterminedthreshold is 5 cmH₂O.

Example 31. The method of any one of Examples 29-30, wherein theprocessor determines the pressure sensor inaccurate when the differenceexceeds the predetermined threshold.

Example 32. The method of any one of Examples 29-31, wherein theprocessor determines the pressure sensor accurate when the difference iswithin the predetermined threshold.

Example 33. The method of any one of Examples 26-32, wherein themeasurement of the pressure, the calculation of the expected pressure,and the comparison of the measured pressure with the expected pressureare performed at a predetermined frequency over a predetermined periodof time.

Example 34. The method of Example 33, wherein the predeterminedfrequency is between 1 and 2 hertz.

Example 35. The method of any one of Examples 33-34, wherein thepredetermined period of time is 5 seconds.

Example 36. The method of any one of Examples 33-35, wherein theprocessor determines the pressure sensor inaccurate based on a pluralityof comparisons between measured pressures and expected pressures.

Example 37. The method of any one of Examples 2-25, wherein thesubordinate characteristic of the respiratory device is a firstsubordinate characteristic, and wherein the processor determines theaccuracy of the pressure sensor by:

-   -   calculating a second subordinate characteristic of the        respiratory device; and    -   comparing the second subordinate characteristic of the        respiratory device with the first subordinate characteristic of        the respiratory device.

Example 38. The method of Example 37, wherein the processor calculatesthe second subordinate characteristic of the respiratory device from ameasured flow rate of the flow of breathable gas, a measured motor speedof the flow generator, and the pressure measured by the pressure sensor.

Example 39. The method of Example 38, wherein the processor calculatesthe second subordinate characteristic of the respiratory device from ameasured temperature of the flow of breathable gas.

Example 40. The method of any one of Examples 37-39, wherein theprocessor determines the accuracy of the pressure sensor by comparing adifference between the first subordinate characteristic and the secondsubordinate characteristic with a predetermined threshold.

Example 41. The method of Example 40, wherein the predeterminedthreshold corresponds with 600 feet.

Example 42. The method of any one of Examples 40-41, wherein theprocessor determines the pressure sensor inaccurate when the differenceexceeds the predetermined threshold.

Example 43. The method of any one of Examples 40-42, wherein theprocessor determines the pressure sensor accurate when the difference iswithin the predetermined threshold.

Example 44. The method of any one of Examples 37-43, wherein theprocessor calculates the second subordinate characteristic at apredetermined frequency for a predetermined period of time.

Example 45. The method of Example 44, wherein the predeterminedfrequency is between 1 and 2 hertz.

Example 46. The method of any one of Examples 44-45, wherein thepredetermined period of time is 5 seconds.

Example 47. The method of any one of Examples 44-46, wherein theprocessor determines the pressure sensor inaccurate when an average ofthe second subordinate characteristic calculated during thepredetermined period of time differs from the first subordinatecharacteristic by an offset greater than a predetermined threshold.

Example 48. The method of any one of Examples 44-47, wherein theprocessor determines the pressure sensor accurate when an average of thesecond subordinate characteristic calculated during the predeterminedperiod of time differs from the first subordinate characteristic by anoffset not greater than a predetermined threshold.

Example 49. The method of any one of Examples 2-48, further comprisingsetting a motor speed for the flow generator based on the measuredpressure, when the processor determines the pressure sensor inaccurate.

Example 50. The method of Example 49, further comprising maintaining themotor speed under a speed limit threshold.

Example 51. The method of any one of Examples 2-48, further comprisingdetermining a desired gas pressure to be generated by the flowgenerator, and determining a desired motor speed for the flow generatorbased on a predetermined association between motor speed values andpressure values.

Example 52. A respiratory apparatus comprising:

-   -   a flow generator with a blower included therein to generate a        flow of breathable gas for delivery to a patient interface at a        pressure above atmospheric pressure;    -   a pressure sensor coupled with the flow generator, the pressure        sensor configured to measure pressure of the flow of breathable        gas; and    -   a processor coupled with the pressure sensor configured to        determine accuracy of the pressure sensor based on the measured        pressure and an subordinate characteristic of the respiratory        apparatus.

Example 53. The respiratory apparatus of Example 52, further comprisinga flow sensor configured to measure a flow rate of the flow ofbreathable gas.

Example 54. The respiratory apparatus of any one of Examples 52-53,further comprising a motor speed sensor configured to measure a motorspeed of the flow generator.

Example 55. The respiratory apparatus of any one of Examples 52-54,further comprising a user I/O device configured to receive thesubordinate characteristic of the respiratory apparatus input by a user.

Example 56. The respiratory apparatus of any one of Examples 52-55,further comprising an altimeter or barometer to determine thesubordinate characteristic of the respiratory apparatus.

Example 57. The respiratory apparatus of any one of Examples 52-56,wherein the processor is configured to calculate the subordinatecharacteristic of the respiratory apparatus.

Example 58. The respiratory apparatus of Example 57, wherein theprocessor determines accuracy of the pressure sensor based on evaluationof the calculated subordinate characteristic.

Example 59. The respiratory apparatus of any one of Examples 57-58,wherein the processor calculates the subordinate characteristic of therespiratory apparatus as a function of (a) the pressure measured by thepressure sensor and one or both of (b1) a measured flow rate of the flowof breathable gas and (b)(2) a measured motor speed of the flowgenerator.

Example 60. The respiratory apparatus of Example 59, wherein theprocessor calculates the subordinate characteristic of the respiratoryapparatus as a function of a measured temperature of the flow ofbreathable gas.

Example 61. The respiratory apparatus of any one of Examples 57-60,wherein the processor calculates the subordinate characteristic of therespiratory apparatus when the flow generator controls the gas at aconstant predetermined flow rate.

Example 62. The respiratory apparatus of Example 61, wherein theconstant predetermined flow rate is 20 liters/minute.

Example 63. The respiratory apparatus of Example 61, wherein theconstant predetermined flow rate is less than 50 liters/minute.

Example 64. The respiratory apparatus of Example 61, wherein theconstant predetermined flow rate is in the range between about 10liters/minute and about 60 liters/minute.

Example 65. The respiratory apparatus of any one of Examples 57-60,wherein the processor calculates the subordinate characteristic of therespiratory apparatus when the flow generator controls a constantpredetermined motor speed.

Example 66. The respiratory apparatus of any one of Examples 58-65,wherein the processor evaluates the calculated subordinatecharacteristic by comparing the calculated subordinate characteristicwith a predetermined range of the subordinate characteristic.

Example 67. The respiratory apparatus of Example 66, wherein thepredetermined range of the subordinate characteristic is a range thatcorresponds with between 0 and 9000 feet above sea level.

Example 68. The respiratory apparatus of claim 66, wherein thepredetermined range of the subordinate characteristic is a range thatcorresponds with between 500 feet below sea level and 10,000 feet abovesea level.

Example 69. The respiratory apparatus of any one of Examples 66-68,wherein the processor deems the calculated subordinate characteristicacceptable when the calculated subordinate characteristic is within thepredetermined range of the subordinate characteristic.

Example 70. The respiratory apparatus of any one of Examples 66-69,wherein the processor deems the calculated subordinate characteristicunacceptable when the calculated subordinate characteristic is outsidethe predetermined range of the subordinate characteristic.

Example 71. The respiratory apparatus of any one of Examples 57-70,wherein the processor calculates the subordinate characteristic of therespiratory apparatus at a predetermined frequency for a predeterminedperiod of time.

Example 72. The respiratory apparatus of claim 71, wherein thepredetermined frequency is between 1 and 2 hertz.

Example 73. The respiratory apparatus of any one of Examples 71-72,wherein the predetermined period of time is 5 seconds.

Example 74. The respiratory apparatus of any one of Examples 71-73,wherein the processor evaluates the pressure sensor based on an averageof the calculated subordinate characteristic.

Example 75. The respiratory apparatus of any one of Examples 71-74,wherein the processor determines the pressure sensor accurate when anaverage of the calculated subordinate characteristic satisfies athreshold comparison.

Example 76. The respiratory apparatus of any one of Examples 52-75,wherein the processor evaluates the pressure sensor in an initializationprocess before the respiratory apparatus provides treatment to apatient.

Example 77. The respiratory apparatus of any one of Examples 52-76,further comprising a memory to store the subordinate characteristic ofthe respiratory apparatus.

Example 78. The respiratory apparatus of any one of Examples 52-77,wherein the processor evaluates accuracy of the pressure sensor by:

-   -   calculating expected pressure of the gas generated by the        respiratory apparatus; and    -   comparing the expected pressure with the pressure measured by        the pressure sensor.

Example 79. The respiratory apparatus of Example 78, wherein theprocessor calculates the expected pressure using the subordinatecharacteristic of the respiratory apparatus, a measured flow rate of theflow of breathable gas, and a measured motor speed of the flowgenerator.

Example 80. The respiratory apparatus of Example 79, wherein theprocessor calculates the expected pressure using a measured temperatureof the flow of breathable gas.

Example 81. The respiratory apparatus of any one of Examples 78-80,wherein the processor determines the accuracy of the pressure sensor bycomparing a difference between the expected pressure and the measuredpressure with a predetermined threshold.

Example 82. The respiratory apparatus of Example 81, wherein thepredetermined threshold is 5 cmH2O.

Example 83. The respiratory apparatus of any one of Examples 81-82,wherein the processor determines the pressure sensor inaccurate when thedifference exceeds the predetermined threshold.

Example 84. The respiratory apparatus of any one of Examples 81-83,wherein the processor determines the pressure sensor accurate when thedifference is within the predetermined threshold.

Example 85. The respiratory apparatus of any one of Examples 78-84,wherein the processor reads measurement of the pressure from thepressure sensor, calculates the expected pressure, and compares theexpected pressure with the measured pressure at a predeterminedfrequency over a predetermined period time of time.

Example 86. The respiratory apparatus of Example 85, wherein thepredetermined frequency is between 1 and 2 hertz.

Example 87. The respiratory apparatus of any one of Examples 85-86,wherein the predetermined period of time is 5 seconds.

Example 88. The respiratory apparatus of any one of Examples 85-87,wherein the processor determines the pressure sensor inaccurate based ona plurality of comparisons between measured pressures and expectedpressures.

Example 89. The respiratory apparatus of any one of Examples 52-77,wherein the subordinate characteristic of the respiratory apparatus is afirst subordinate characteristic, and wherein the processor determinesthe accuracy of the pressure sensor by:

-   -   calculating a second subordinate characteristic of the        respiratory apparatus; and    -   comparing the second subordinate characteristic of the        respiratory apparatus with the first subordinate characteristic        of the respiratory apparatus.

Example 90. The respiratory apparatus of Example 89, wherein theprocessor calculates the second subordinate characteristic of therespiratory apparatus using a measured flow rate of the flow ofbreathable gas, a measured motor speed of the flow generator, and apressure measured by the pressure sensor.

Example 91. The respiratory apparatus of Example 90, wherein theprocessor calculates the second subordinate characteristic of therespiratory apparatus from a measured temperature of the flow ofbreathable gas.

Example 92. The respiratory apparatus of any one of Examples 89-91,wherein the processor determines the accuracy of the pressure sensor bycomparing a difference between the first subordinate characteristic andthe second subordinate characteristic with a predetermined threshold.

Example 93. The respiratory apparatus of Example 92, wherein thepredetermined threshold is 600 feet.

Example 94. The respiratory apparatus of any one of Examples 92-93,wherein the processor determines the pressure sensor inaccurate when thedifference exceeds the predetermined threshold.

Example 95. The respiratory apparatus of any one of Examples 92-94,wherein the processor determines the pressure sensor accurate when thedifference is within the predetermined threshold.

Example 96. The respiratory apparatus of any one of Examples 89-95,wherein the processor is configure to read a measurement of the pressurefrom the pressure sensor and to calculate the second subordinatecharacteristic based on the measurement at a predetermined frequencyover a predetermined period of time.

Example 97. The respiratory apparatus of Example 96, wherein thepredetermined frequency is between 1 and 2 hertz.

Example 98. The respiratory apparatus of any one of Examples 96-97,wherein the predetermined period of time is 5 seconds.

Example 99. The respiratory apparatus of any one of Examples 96-98,wherein the processor determines the pressure sensor inaccurate when anaverage of the second subordinate characteristic calculated during thepredetermined period of time differs from the first subordinatecharacteristic by an offset greater than a predetermined threshold.

Example 100. The respiratory apparatus of any one of Examples 96-99,wherein the pressure sensor is determined accurate when an average ofthe second subordinate characteristic calculate over the predeterminedperiod of time differs from the first subordinate characteristic by anoffset not greater than a predetermined threshold.

Example 101. The respiratory apparatus of any one of Examples 52-100,wherein the processor is further configured to set the flow generator toa motor speed based on the pressure measured by the pressure sensor,when the processor determines the pressure sensor inaccurate.

Example 102. The respiratory apparatus of Example 101, wherein theprocessor is further configured to maintain the motor speed under aspeed limit threshold.

Example 103. The respiratory apparatus of any one of Examples 52-100,wherein the processor is further configured to determine a desired gaspressure to be generated by the flow generator, and determine a desiredmotor speed for the flow generator based on a predetermined associationbetween motor speed values and pressure values.

Example 104. The method or apparatus of any one of the precedingexamples wherein the subordinate characteristic is altitude.

Example 105. The method or apparatus of any one of the precedingexamples wherein the subordinate characteristic is atmospheric pressurein which the apparatus is operated.

Example 106. The method or apparatus of any one of the precedingexamples wherein the subordinate characteristic is density altitude.

Example 107. The method or apparatus of any one of the precedingexamples wherein the subordinate characteristic is pressure altitude.

1. A respiratory apparatus configured to provide a respiratorytreatment, the respiratory apparatus comprising: a flow generator havinga blower to generate a flow of breathable gas for a patient interface ata pressure above atmospheric pressure; a pressure sensor configured tomeasure the pressure; a controller coupled to the pressure sensor andthe flow generator, the controller configured to control a delivery ofrespiratory treatment from the flow generator in a first mode accordingto a pressure control loop with a signal from the pressure sensor; thecontroller further configured to: test accuracy of the pressure sensor;upon detection that the pressure sensor is inaccurate, change control ofthe delivery of the respiratory treatment with the flow generator fromthe first mode to an operational safe mode; and thereafter controldelivery of respiratory treatment according to the operational safemode.
 2. The respiratory apparatus of claim 1, wherein the operationalsafe mode is a pressure controlled mode, and wherein the controller isfurther configured to: determine a desired pressure level setting offlow generator; set a motor speed threshold; obtain a pressure readingfrom the pressure sensor; and increase a motor speed of the flowgenerator to a next motor speed if (i) the pressure reading does notexceed the desired pressure level and (ii) the next motor speed does notexceed a maximum motor speed.
 3. The respiratory apparatus of claim 2,wherein the maximum motor speed is a motor speed corresponding to thedesired pressure level.
 4. The respiratory apparatus of claim 2, whereinthe maximum motor speed is a motor speed corresponding to a maximumpressure level.
 5. The respiratory apparatus of claim 2, wherein thecontroller is configured to repeatedly increase a motor speed of theflow generator to a next motor speed unless increasing the motor speed(i) causes the pressure reading to exceed the desired pressure level or(ii) causes the next motor speed to exceed the maximum motor speed. 6.The respiratory apparatus of claim 1, wherein the operational safe modeis a speed controlled mode, and wherein the controller is furtherconfigured to: determine a desired pressure level setting of the flowgenerator; determine a desired motor speed based on the desired pressurelevel; and control delivery of respiratory treatment at the desiredmotor speed.
 7. The respiratory apparatus of claim 6 further comprisinga memory containing a look up table for defining one-on-one mappingsbetween a plurality of different motor speeds and a plurality ofdifferent pressure levels, and wherein the controller is configured todetermine the desired motor speed based on the look up table.
 8. Therespiratory apparatus of claim 6, wherein the desired pressure level isa constant pressure value maintained throughout a session of therespiratory treatment.
 9. The respiratory apparatus of claim 6, whereinthe desired pressure varies throughout a session of the respiratorytreatment based on one or more detected breathing conditions.
 10. Therespiratory apparatus of claim 1, wherein the controller is configuredto test the accuracy of the pressure sensor based on based a measuredpressure and an altitude of the respiratory apparatus.
 11. Therespiratory apparatus of claim 10, wherein the controller is configuredto calculate the altitude of the respiratory apparatus as a function of(a) the pressure measured by the pressure sensor and one or both of (b1)a measured flow rate of the flow of breathable gas and (b2) a measuredmotor speed of the flow generator.
 12. The respiratory apparatus ofclaim 11, wherein the controller is configured to calculate the altitudeof the respiratory apparatus as a function of a measured temperature ofthe flow of breathable gas.
 13. The respiratory apparatus of claim 11,wherein the controller is configured to test the accuracy of thepressure sensor based on an average of calculated altitudes.
 14. Therespiratory apparatus of claim 13, wherein the controller determines thepressure sensor is accurate when the average of calculated altitudessatisfies a threshold comparison.
 15. The respiratory apparatus of claim1, wherein the controller is configured to test the accuracy of thepressure sensor by: calculating expected pressure of the gas generatedby the respiratory apparatus; and comparing the expected pressure withthe pressure measured by the pressure sensor.
 16. The respiratoryapparatus of claim 15, wherein the controller is configured to calculatethe expected pressure using an altitude of the respiratory apparatus, ameasured flow rate of the flow of breathable gas, and a measured motorspeed of the flow generator.
 17. The respiratory apparatus of claim 16,wherein the controller is configured to calculate the expected pressureusing a measured temperature of the flow of breathable gas.
 18. Therespiratory apparatus of claim 16, wherein the controller is configuredto detect that the pressure sensor is inaccurate when a differencebetween the expected pressure and the pressure measured by the pressuresensor exceeds a predetermined threshold.
 19. The respiratory apparatusof claim 18, wherein the predetermined threshold is 5 cmH2O.
 20. Therespiratory apparatus of claim 15, wherein the controller is configuredto read measurement of pressure from the pressure sensor, calculate theexpected pressure, and compare the expected pressure with the measuredpressure at a predetermined frequency over a predetermined period timeof time.
 21. The respiratory apparatus of claim 20, wherein thecontroller is configured to detect that the pressure sensor isinaccurate based on a plurality of comparisons between measuredpressures and expected pressures.
 22. The respiratory apparatus of claim10, wherein the altitude of the respiratory apparatus is a firstaltitude, and wherein the controller is configured to test accuracy ofthe pressure sensor by: calculating a second altitude of the respiratoryapparatus using a measured flow rate of the flow of breathable gas, ameasured motor speed of the flow generator, and the pressure measured bythe pressure sensor; and comparing the second altitude of therespiratory apparatus with the first altitude of the respiratoryapparatus.
 23. The respiratory apparatus of claim 22, wherein thecontroller is configured to detect that the pressure sensor isinaccurate when a difference between the first altitude and the secondaltitude exceeds a predetermined threshold.
 24. The respiratoryapparatus of claim 23, wherein the predetermined threshold is 600 feet.25. The respiratory apparatus of claim 22, wherein the controller isconfigured to read a measurement of the pressure from the pressuresensor and to calculate the second altitude based on the measurement ata predetermined frequency over a predetermined period of time.
 26. Therespiratory apparatus of claim 25, wherein the controller is configuredto detect that the pressure sensor is inaccurate when an average of thesecond altitude calculated during the predetermined period of timediffers from the first altitude by an offset greater than apredetermined threshold.
 27. A control method in respiratory apparatusfor a respiratory treatment comprising: controlling settings ofrespiratory treatment with a flow generator in a first mode according toa pressure control loop with a signal from a pressure sensor; testingaccuracy of the pressure sensor; upon detection that the pressure sensoris inaccurate, changing mode of operations for the respiratory treatmentwith the flow generator from the first mode to an operational safe mode;and thereafter controlling operations according to the operational safemode.
 28. The control method of claim 27, wherein the operational safemode is a pressure controlled mode, and wherein the control methodfurther comprises: determining a desired pressure level setting for theflow generator; setting a motor speed threshold; obtaining a pressurereading from the pressure sensor; and increasing a motor speed of theflow generator to a next motor speed if (i) the pressure reading doesnot exceed the desired pressure level and (ii) the next motor speed doesnot exceed a maximum motor speed.
 29. The control method of claim 28,wherein the maximum motor speed is a motor speed corresponding to thedesired pressure level.
 30. The control method of claim 28, wherein themaximum motor speed is a motor speed corresponding to a maximum pressurelevel.
 31. The control method of claim 28, wherein the control methodfurther comprises repeatedly increasing a motor speed of the flowgenerator to a next motor speed unless increasing the motor speed (i)causes the pressure reading to exceed the desired pressure level or (ii)causes the next motor speed to exceed the maximum motor speed.
 32. Thecontrol method of claim 27, wherein the operational safe mode is a speedcontrolled mode, and wherein the control method further comprises:determining a desired pressure level setting of the flow generator;determining a desired motor speed based on the desired pressure level;and controlling delivery of respiratory treatment at the desired motorspeed.
 33. The control method of claim 32, wherein determining thedesired motor speed is further based on a look up table definingone-on-one mappings between a plurality of different motor speeds and aplurality of different pressure levels.
 34. The control method of claim32, wherein the desired pressure level is a constant pressure valuemaintained throughout a session of the respiratory treatment.
 35. Thecontrol method of claim 32, wherein the desired pressure variesthroughout a session of the respiratory treatment based on one or moredetected breathing conditions.
 36. The control method of claim 27,wherein testing the accuracy of the pressure sensor is based on based ameasured pressure and an altitude of the respiratory apparatus.
 37. Thecontrol method of claim 36, wherein the control method further comprisescalculating the altitude of the respiratory apparatus as a function of(a) the pressure measured by the pressure sensor and one or both of (b1)a measured flow rate of a flow of breathable gas and (b2) a measuredmotor speed of the flow generator.
 38. The control method of claim 37,wherein the control method further comprises calculating the altitude ofthe respiratory apparatus as a function of a measured temperature of theflow of breathable gas.
 39. The control method of claim 37, wherein thecontrol method further comprises testing the accuracy of the pressuresensor based on an average of the calculated altitudes.
 40. The controlmethod of claim 39, wherein the control method further comprisesdetermining the pressure sensor is accurate when an average of thecalculated altitudes satisfies a threshold comparison.
 41. The controlmethod of claim 37, wherein the control method further comprises testingthe accuracy of the pressure sensor by: calculating expected pressure ofgas generated by the respiratory apparatus; and comparing the expectedpressure with the pressure measured by the pressure sensor.
 42. Thecontrol method of claim 41, wherein the control method further comprisescalculating the expected pressure using an altitude of the respiratoryapparatus, a measured flow rate of the flow of breathable gas, and ameasured motor speed of the flow generator.
 43. The control method ofclaim 42, wherein the control method further comprises calculating theexpected pressure using a measured temperature of the flow of breathablegas.
 44. The control method of claim 42, wherein the control methodfurther comprises detecting that the pressure sensor is inaccurate whena difference between the expected pressure and the pressure measured bythe pressure sensor exceeds a predetermined threshold.
 45. The controlmethod of claim 44, wherein the predetermined threshold is 5 cmH2O. 46.The control method of claim 41, wherein the control method furthercomprises reading measurement of pressure from the pressure sensor,calculating the expected pressure, and comparing the expected pressurewith the measured pressure at a predetermined frequency over apredetermined period time of time.
 47. The control method of claim 46,wherein the control method further comprises detecting that the pressuresensor is inaccurate based on a plurality of comparisons betweenmeasured pressures and expected pressures.
 48. The control method ofclaim 46, wherein the altitude of the respiratory apparatus is a firstaltitude, and wherein the control method further comprises testingaccuracy of the pressure sensor by: calculating a second altitude of therespiratory apparatus using a measured flow rate of the flow ofbreathable gas, a measured motor speed of the flow generator, and thepressure measured by the pressure sensor; and comparing the secondaltitude of the respiratory apparatus with the first altitude of therespiratory apparatus.
 49. The control method of claim 48, wherein thecontrol method further comprises detecting that the pressure sensor isinaccurate when a difference between the first altitude and the secondaltitude exceeds a predetermined threshold.
 50. The control method ofclaim 49, wherein the predetermined threshold is 600 feet.
 51. Thecontrol method of claim 48, wherein the control method further comprisesreading a measurement of the pressure from the pressure sensor andcalculating the second altitude based on the measurement at apredetermined frequency over a predetermined period of time.
 52. Thecontrol method of claim 51, wherein the control method further comprisesdetecting that the pressure sensor is inaccurate when an average of thesecond altitude calculated during the predetermined period of timediffers from the first altitude by an offset greater than apredetermined threshold